SQL UNIQUE Constraint on CREATE TABLE
The following SQL creates a UNIQUE constraint on the "P_Id" column
when the "Persons" table is created:
MySQL:
|
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
UNIQUE (P_Id)
)
|
SQL Server / Oracle / MS Access:
|
CREATE TABLE Persons
(
P_Id int NOT NULL UNIQUE,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
|
To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint
on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
|
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
)
|
SQL UNIQUE Constraint on ALTER TABLE
To create a UNIQUE constraint on the "P_Id" column when the table
is already created, use the following SQL:
MySQL / SQL Server / Oracle / MS Access:
|
ALTER TABLE Persons
ADD UNIQUE (P_Id)
|
To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint
on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
|
ALTER TABLE Persons
ADD CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
|
To DROP a UNIQUE Constraint
To drop a UNIQUE constraint, use the following SQL:
MySQL:
|
ALTER TABLE Persons
DROP INDEX uc_PersonID
|
SQL Server / Oracle / MS Access:
|
ALTER TABLE Persons
DROP CONSTRAINT uc_PersonID
|
SQL PRIMARY KEY Constraint
SQL PRIMARY KEY Constraint
The PRIMARY KEY constraint uniquely identifies each record in a database
table.
Primary keys must contain unique values.
A primary key column cannot contain NULL values.
Each table should have a primary key, and each table can have only ONE
primary key.
SQL PRIMARY KEY Constraint on CREATE TABLE
The following SQL creates a PRIMARY KEY on the "P_Id" column when
the "Persons" table is created:
MySQL:
|
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
PRIMARY KEY (P_Id)
)
|
SQL Server / Oracle / MS Access:
|
CREATE TABLE Persons
(
P_Id int NOT NULL PRIMARY KEY,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
|
To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY
constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
|
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
)
|
SQL PRIMARY KEY Constraint on ALTER TABLE
To create a PRIMARY KEY constraint on the "P_Id" column when the
table is already created, use the following SQL:
MySQL / SQL Server / Oracle / MS Access:
|
ALTER TABLE Persons
ADD PRIMARY KEY (P_Id)
|
To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY
constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
|
ALTER TABLE Persons
ADD CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
|
Note: If you use the ALTER TABLE statement to add a primary key, the
primary key column(s) must already have been declared to not contain NULL
values (when the table was first created).
To DROP a PRIMARY KEY Constraint
To drop a PRIMARY KEY constraint, use the following SQL:
MySQL:
|
ALTER TABLE Persons
DROP PRIMARY KEY
|
SQL Server / Oracle / MS Access:
|
ALTER TABLE Persons
DROP CONSTRAINT pk_PersonID
|
SQL FOREIGN KEY Constraint
SQL FOREIGN KEY Constraint
A FOREIGN KEY in one table points to a PRIMARY KEY in another table.
Let's illustrate the foreign key with an example. Look at the following two
tables:
The "Persons" table:
|
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
The "Orders" table:
|
O_Id
|
OrderNo
|
P_Id
|
|
1
|
77895
|
3
|
|
2
|
44678
|
3
|
|
3
|
22456
|
2
|
|
4
|
24562
|
1
|
Note that the "P_Id" column in the "Orders" table points
to the "P_Id" column in the "Persons" table.
The "P_Id" column in the "Persons" table is the PRIMARY
KEY in the "Persons" table.
The "P_Id" column in the "Orders" table is a FOREIGN KEY
in the "Orders" table.
The FOREIGN KEY constraint is used to prevent actions that would destroy
links between tables.
The FOREIGN KEY constraint also prevents that invalid data form being
inserted into the foreign key column, because it has to be one of the values
contained in the table it points to.
SQL FOREIGN KEY Constraint on CREATE TABLE
The following SQL creates a FOREIGN KEY on the "P_Id" column when
the "Orders" table is created:
MySQL:
|
CREATE TABLE Orders
(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
PRIMARY KEY (O_Id),
FOREIGN KEY (P_Id) REFERENCES Persons(P_Id)
)
|
SQL Server / Oracle / MS Access:
|
CREATE TABLE Orders
(
O_Id int NOT NULL PRIMARY KEY,
OrderNo int NOT NULL,
P_Id int FOREIGN KEY REFERENCES Persons(P_Id)
)
|
To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY
constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
|
CREATE TABLE Orders
(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
PRIMARY KEY (O_Id),
CONSTRAINT fk_PerOrders FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)
)
|
SQL FOREIGN KEY Constraint on ALTER TABLE
To create a FOREIGN KEY constraint on the "P_Id" column when the
"Orders" table is already created, use the following SQL:
MySQL / SQL Server / Oracle / MS Access:
|
ALTER TABLE Orders
ADD FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)
|
To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY
constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
|
ALTER TABLE Orders
ADD CONSTRAINT fk_PerOrders
FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)
|
To DROP a FOREIGN KEY Constraint
To drop a FOREIGN KEY constraint, use the following SQL:
MySQL:
|
ALTER TABLE Orders
DROP FOREIGN KEY fk_PerOrders
|
SQL Server / Oracle / MS Access:
|
ALTER TABLE Orders
DROP CONSTRAINT fk_PerOrders
|
SQL CHECK Constraint
SQL CHECK Constraint
The CHECK constraint is used to limit the value range that can be placed in
a column.
If you define a CHECK constraint on a single column it allows only certain
values for this column.
If you define a CHECK constraint on a table it can limit the values in
certain columns based on values in other columns in the row.
SQL CHECK Constraint on CREATE TABLE
The following SQL creates a CHECK constraint on the "P_Id" column
when the "Persons" table is created. The CHECK constraint specifies
that the column "P_Id" must only include integers greater than 0.
My SQL:
|
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CHECK (P_Id>0)
)
|
SQL Server / Oracle / MS Access:
|
CREATE TABLE Persons
(
P_Id int NOT NULL CHECK (P_Id>0),
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
|
To allow naming of a CHECK constraint, and for defining a CHECK constraint
on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
|
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT chk_Person CHECK (P_Id>0 AND City='Sandnes')
)
|
SQL CHECK Constraint on ALTER TABLE
To create a CHECK constraint on the "P_Id" column when the table
is already created, use the following SQL:
MySQL / SQL Server / Oracle / MS Access:
|
ALTER TABLE Persons
ADD CHECK (P_Id>0)
|
To allow naming of a CHECK constraint, and for defining a CHECK constraint
on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
|
ALTER TABLE Persons
ADD CONSTRAINT chk_Person CHECK (P_Id>0 AND City='Sandnes')
|
To DROP a CHECK Constraint
To drop a CHECK constraint, use the following SQL:
SQL Server / Oracle / MS Access:
|
ALTER TABLE Persons
DROP CONSTRAINT chk_Person
|
SQL DEFAULT Constraint
SQL DEFAULT Constraint
The DEFAULT constraint is used to insert a default value into a column.
The default value will be added to all new records, if no other value is
specified.
SQL DEFAULT Constraint on CREATE TABLE
The following SQL creates a DEFAULT constraint on the "City"
column when the "Persons" table is created:
My SQL / SQL Server / Oracle / MS Access:
|
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255) DEFAULT 'Sandnes'
)
|
The DEFAULT constraint can also be used to insert system values, by using
functions like GETDATE():
|
CREATE TABLE Orders
(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
OrderDate date DEFAULT GETDATE()
)
|
SQL DEFAULT Constraint on ALTER TABLE
To create a DEFAULT constraint on the "City" column when the table
is already created, use the following SQL:
MySQL:
|
ALTER TABLE Persons
ALTER City SET DEFAULT 'SANDNES'
|
SQL Server / Oracle / MS Access:
|
ALTER TABLE Persons
ALTER COLUMN City SET DEFAULT 'SANDNES'
|
To DROP a DEFAULT Constraint
To drop a DEFAULT constraint, use the following SQL:
MySQL:
|
ALTER TABLE Persons
ALTER City DROP DEFAULT
|
SQL Server / Oracle / MS Access:
|
ALTER TABLE Persons
ALTER COLUMN City DROP DEFAULT
|
SQL CREATE INDEX Statement
The CREATE INDEX statement is used to create indexes in tables.
Indexes allow the database application to find data fast;
without reading the whole table.
Indexes
An index can be created in a table to find data more quickly and
efficiently.
The users cannot see the indexes, they are just used to speed up
searches/queries.
Note: Updating a table with indexes takes more time than updating a
table without (because the indexes also need an update). So you should only
create indexes on columns (and tables) that will be frequently searched
against.
SQL CREATE INDEX Syntax
Creates an index on a table. Duplicate values are allowed:
|
CREATE INDEX index_name
ON table_name (column_name)
|
SQL CREATE UNIQUE INDEX Syntax
Creates a unique index on a table. Duplicate values are not allowed:
|
CREATE UNIQUE INDEX
index_name
ON table_name (column_name)
|
Note: The syntax for creating indexes varies amongst different
databases. Therefore: Check the syntax for creating indexes in your database.
CREATE INDEX Example
The SQL statement below creates an index named "PIndex" on the
"LastName" column in the "Persons" table:
|
CREATE INDEX PIndex
ON Persons (LastName)
|
If you want to create an index on a combination of columns, you can list the
column names within the parentheses, separated by commas:
|
CREATE INDEX PIndex
ON Persons (LastName, FirstName)
|
SQL DROP INDEX, DROP TABLE, and DROP DATABASE
Indexes, tables, and databases can easily be deleted/removed
with the DROP statement.
The DROP INDEX Statement
The DROP INDEX statement is used to delete an index in a table.
DROP INDEX Syntax for MS Access:
|
DROP INDEX index_name ON
table_name
|
DROP INDEX Syntax for MS SQL Server:
|
DROP INDEX
table_name.index_name
|
DROP INDEX Syntax for DB2/Oracle:
DROP INDEX Syntax for MySQL:
|
ALTER TABLE table_name DROP
INDEX index_name
|
The DROP TABLE Statement
The DROP TABLE statement is used to delete a table.
The DROP DATABASE Statement
The DROP DATABASE statement is used to delete a database.
|
DROP DATABASE database_name
|
The TRUNCATE TABLE Statement
What if we only want to delete the data inside the table, and not the table
itself?
Then, use the TRUNCATE TABLE statement:
|
TRUNCATE TABLE table_name
|
SQL ALTER TABLE Statement
The ALTER TABLE Statement
The ALTER TABLE statement is used to add, delete, or modify columns in an
existing table.
SQL ALTER TABLE Syntax
To add a column in a table, use the following syntax:
|
ALTER TABLE table_name
ADD column_name datatype
|
To delete a column in a table, use the following syntax (notice that some
database systems don't allow deleting a column):
|
ALTER TABLE table_name
DROP COLUMN column_name
|
To change the data type of a column in a table, use the following syntax:
|
ALTER TABLE table_name
ALTER COLUMN column_name datatype
|
SQL ALTER TABLE Example
Look at the "Persons" table:
|
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
Now we want to add a column named "DateOfBirth" in the
"Persons" table.
We use the following SQL statement:
|
ALTER TABLE Persons
ADD DateOfBirth date
|
Notice that the new column, "DateOfBirth", is of type date and is
going to hold a date. The data type specifies what type of data the column can
hold. For a complete reference of all the data types available in MS Access,
MySQL, and SQL Server, go to our complete
Data Types reference.
The "Persons" table will now like this:
|
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
DateOfBirth
|
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
|
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
|
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
|
Change Data Type Example
Now we want to change the data type of the column named
"DateOfBirth" in the "Persons" table.
We use the following SQL statement:
|
ALTER TABLE Persons
ALTER COLUMN DateOfBirth year
|
Notice that the "DateOfBirth" column is now of type year and is
going to hold a year in a two-digit or four-digit format.
DROP COLUMN Example
Next, we want to delete the column named "DateOfBirth" in the
"Persons" table.
We use the following SQL statement:
|
ALTER TABLE Persons
DROP COLUMN DateOfBirth
|
The "Persons" table will now like this:
|
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
SQL AUTO INCREMENT Field
Auto-increment allows a unique number to be generated when a new
record is inserted into a table.
AUTO INCREMENT a Field
Very often we would like the value of the primary key field to be created
automatically every time a new record is inserted.
We would like to create an auto-increment field in a table.
Syntax for MySQL
The following SQL statement defines the "P_Id" column to be an
auto-increment primary key field in the "Persons" table:
|
CREATE TABLE Persons
(
P_Id int NOT NULL AUTO_INCREMENT,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
PRIMARY KEY (P_Id)
)
|
MySQL uses the AUTO_INCREMENT keyword to perform an auto-increment feature.
By default, the starting value for AUTO_INCREMENT is 1, and it will
increment by 1 for each new record.
To let the AUTO_INCREMENT sequence start with another value, use the
following SQL statement:
|
ALTER TABLE Persons
AUTO_INCREMENT=100
|
To insert a new record into the "Persons" table, we will not have
to specify a value for the "P_Id" column (a unique value will be
added automatically):
|
INSERT INTO Persons
(FirstName,LastName)
VALUES ('Lars','Monsen')
|
The SQL statement above would insert a new record into the
"Persons" table. The "P_Id" column would be assigned a
unique value. The "FirstName" column would be set to "Lars"
and the "LastName" column would be set to "Monsen".
Syntax for SQL Server
The following SQL statement defines the "P_Id" column to be an
auto-increment primary key field in the "Persons" table:
|
CREATE TABLE Persons
(
P_Id int PRIMARY KEY IDENTITY,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
|
The MS SQL Server uses the IDENTITY keyword to perform an auto-increment
feature.
By default, the starting value for IDENTITY is 1, and it will increment by 1
for each new record.
To specify that the "P_Id" column should start at value 10 and
increment by 5, change the identity to IDENTITY(10,5).
To insert a new record into the "Persons" table, we will not have
to specify a value for the "P_Id" column (a unique value will be
added automatically):
|
INSERT INTO Persons
(FirstName,LastName)
VALUES ('Lars','Monsen')
|
The SQL statement above would insert a new record into the
"Persons" table. The "P_Id" column would be assigned a
unique value. The "FirstName" column would be set to "Lars"
and the "LastName" column would be set to "Monsen".
Syntax for Access
The following SQL statement defines the "P_Id" column to be an
auto-increment primary key field in the "Persons" table:
|
CREATE TABLE Persons
(
P_Id PRIMARY KEY AUTOINCREMENT,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
|
The MS Access uses the AUTOINCREMENT keyword to perform an auto-increment
feature.
By default, the starting value for AUTOINCREMENT is 1, and it will increment
by 1 for each new record.
To specify that the "P_Id" column should start at value 10 and
increment by 5, change the autoincrement to AUTOINCREMENT(10,5).
To insert a new record into the "Persons" table, we will not have
to specify a value for the "P_Id" column (a unique value will be
added automatically):
|
INSERT INTO Persons
(FirstName,LastName)
VALUES ('Lars','Monsen')
|
The SQL statement above would insert a new record into the
"Persons" table. The "P_Id" column would be assigned a
unique value. The "FirstName" column would be set to "Lars"
and the "LastName" column would be set to "Monsen".
Syntax for Oracle
In Oracle the code is a little bit more tricky.
You will have to create an auto-increment field with the sequence object (this
object generates a number sequence).
Use the following CREATE SEQUENCE syntax:
|
CREATE SEQUENCE seq_person
MINVALUE 1
START WITH 1
INCREMENT BY 1
CACHE 10
|
The code above creates a sequence object called seq_person, that starts with
1 and will increment by 1. It will also cache up to 10 values for performance.
The cache option specifies how many sequence values will be stored in memory
for faster access.
To insert a new record into the "Persons" table, we will have to
use the nextval function (this function retrieves the next value from
seq_person sequence):
|
INSERT INTO Persons
(P_Id,FirstName,LastName)
VALUES (seq_person.nextval,'Lars','Monsen')
|
The SQL statement above would insert a new record into the
"Persons" table. The "P_Id" column would be assigned the
next number from the seq_person sequence. The "FirstName" column
would be set to "Lars" and the "LastName" column would be
set to "Monsen".
SQL Views
A view is a virtual table.
This chapter shows how to create, update, and delete a view.
SQL CREATE VIEW Statement
In SQL, a view is a virtual table based on the result-set of an SQL
statement.
A view contains rows and columns, just like a real table. The fields in a
view are fields from one or more real tables in the database.
You can add SQL functions, WHERE, and JOIN statements to a view and present
the data as if the data were coming from one single table.
SQL CREATE VIEW Syntax
|
CREATE VIEW view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition
|
Note: A view always shows up-to-date data! The database engine
recreates the data, using the view's SQL statement, every time a user queries a
view.
SQL CREATE VIEW Examples
If you have the Northwind database you can see that it has several views
installed by default.
The view "Current Product List" lists all active products
(products that are not discontinued) from the "Products" table. The
view is created with the following SQL:
|
CREATE VIEW [Current Product
List] AS
SELECT ProductID,ProductName
FROM Products
WHERE Discontinued=No
|
We can query the view above as follows:
|
SELECT * FROM [Current
Product List]
|
Another view in the Northwind sample database selects every product in the
"Products" table with a unit price higher than the average unit
price:
|
CREATE VIEW [Products Above
Average Price] AS
SELECT ProductName,UnitPrice
FROM Products
WHERE UnitPrice>(SELECT AVG(UnitPrice) FROM Products)
|
We can query the view above as follows:
|
SELECT * FROM [Products
Above Average Price]
|
Another view in the Northwind database calculates the total sale for each
category in 1997. Note that this view selects its data from another view called
"Product Sales for 1997":
|
CREATE VIEW [Category Sales
For 1997] AS
SELECT DISTINCT CategoryName,Sum(ProductSales) AS CategorySales
FROM [Product Sales for 1997]
GROUP BY CategoryName
|
We can query the view above as follows:
|
SELECT * FROM [Category
Sales For 1997]
|
We can also add a condition to the query. Now we want to see the total sale
only for the category "Beverages":
|
SELECT * FROM [Category
Sales For 1997]
WHERE CategoryName='Beverages'
|
SQL Updating a View
You can update a view by using the following syntax:
SQL CREATE OR REPLACE VIEW Syntax
|
CREATE OR REPLACE VIEW
view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition
|
Now we want to add the "Category" column to the "Current
Product List" view. We will update the view with the following SQL:
|
CREATE VIEW [Current Product
List] AS
SELECT ProductID,ProductName,Category
FROM Products
WHERE Discontinued=No
|
SQL Dropping a View
You can delete a view with the DROP VIEW command.
SQL DROP VIEW Syntax
SQL Date Functions
SQL Dates
The most difficult part when working with
dates is to be sure that the format of the date you are trying to insert,
matches the format of the date column in the database.
As long as your data contains only the date portion, your queries will work
as expected. However, if a time portion is involved, it gets complicated.
Before talking about the complications of querying for dates, we will look
at the most important built-in functions for working with dates.
MySQL Date Functions
The following table lists the most important built-in date functions in
MySQL:
|
Function
|
Description
|
|
|
Returns the current date and
time
|
|
|
Returns the current date
|
|
|
Returns the current time
|
|
|
Extracts the date part of a
date or date/time expression
|
|
|
Returns a single part of a
date/time
|
|
|
Adds a specified time
interval to a date
|
|
|
Subtracts a specified time
interval from a date
|
|
|
Returns the number of days
between two dates
|
|
|
Displays date/time data in
different formats
|
SQL Server Date Functions
The following table lists the most important built-in date functions in SQL
Server:
|
Function
|
Description
|
|
|
Returns the current date and
time
|
|
|
Returns a single part of a
date/time
|
|
|
Adds or subtracts a
specified time interval from a date
|
|
|
Returns the time between two
dates
|
|
|
Displays date/time data in
different formats
|
SQL Date Data Types
MySQL comes with the following data types for storing a date or a
date/time value in the database:
·
DATE - format YYYY-MM-DD
·
DATETIME - format: YYYY-MM-DD HH:MM:SS
·
TIMESTAMP - format: YYYY-MM-DD HH:MM:SS
·
YEAR - format YYYY or YY
SQL Server comes with the following data types for storing a date or
a date/time value in the database:
·
DATE - format YYYY-MM-DD
·
DATETIME - format: YYYY-MM-DD HH:MM:SS
·
SMALLDATETIME - format: YYYY-MM-DD HH:MM:SS
·
TIMESTAMP - format: a unique number
Note: The date types are chosen for a column when you create a new
table in your database!
For an overview of all data types available, go to our complete
Data Types reference.
SQL Working with Dates
You can compare two dates easily if there is
no time component involved!
Assume we have the following "Orders" table:
|
OrderId
|
ProductName
|
OrderDate
|
|
1
|
Geitost
|
2008-11-11
|
|
2
|
Camembert Pierrot
|
2008-11-09
|
|
3
|
Mozzarella di Giovanni
|
2008-11-11
|
|
4
|
Mascarpone Fabioli
|
2008-10-29
|
Now we want to select the records with an OrderDate of
"2008-11-11" from the table above.
We use the following SELECT statement:
|
SELECT * FROM Orders WHERE
OrderDate='2008-11-11'
|
The result-set will look like this:
|
OrderId
|
ProductName
|
OrderDate
|
|
1
|
Geitost
|
2008-11-11
|
|
3
|
Mozzarella di Giovanni
|
2008-11-11
|
Now, assume that the "Orders" table looks like this (notice the
time component in the "OrderDate" column):
|
OrderId
|
ProductName
|
OrderDate
|
|
1
|
Geitost
|
2008-11-11 13:23:44
|
|
2
|
Camembert Pierrot
|
2008-11-09 15:45:21
|
|
3
|
Mozzarella di Giovanni
|
2008-11-11 11:12:01
|
|
4
|
Mascarpone Fabioli
|
2008-10-29 14:56:59
|
If we use the same SELECT statement as above:
|
SELECT * FROM Orders WHERE
OrderDate='2008-11-11'
|
we will get no result! This is because the query is looking only for dates
with no time portion.
Tip: If you want to keep your queries simple and easy to maintain, do
not allow time components in your dates!
SQL NULL Values
NULL values represent missing unknown data.
By default, a table column can hold NULL values.
This chapter will explain the IS NULL and IS NOT NULL operators.
SQL NULL Values
If a column in a table is optional, we can insert a new record or update an
existing record without adding a value to this column. This means that the
field will be saved with a NULL value.
NULL values are treated differently from other values.
NULL is used as a placeholder for unknown or inapplicable values.
Note: It is not possible to compare
NULL and 0; they are not equivalent.
SQL Working with NULL Values
Look at the following "Persons" table:
|
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
|
1
|
Hansen
|
Ola
|
|
Sandnes
|
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
|
3
|
Pettersen
|
Kari
|
|
Stavanger
|
Suppose that the "Address" column in the "Persons" table
is optional. This means that if we insert a record with no value for the
"Address" column, the "Address" column will be saved with a
NULL value.
How can we test for NULL values?
It is not possible to test for NULL values with comparison operators, such
as =, <, or <>.
We will have to use the IS NULL and IS NOT NULL operators instead.
SQL IS NULL
How do we select only the records with NULL values in the
"Address" column?
We will have to use the IS NULL operator:
|
SELECT
LastName,FirstName,Address FROM Persons
WHERE Address IS NULL
|
The result-set will look like this:
|
LastName
|
FirstName
|
Address
|
|
Hansen
|
Ola
|
|
|
Pettersen
|
Kari
|
|
Tip: Always use IS NULL to look for
NULL values.
SQL IS NOT NULL
How do we select only the records with no NULL values in the
"Address" column?
We will have to use the IS NOT NULL operator:
|
SELECT
LastName,FirstName,Address FROM Persons
WHERE Address IS NOT NULL
|
The result-set will look like this:
|
LastName
|
FirstName
|
Address
|
|
Svendson
|
Tove
|
Borgvn 23
|
In the next chapter we will look at the ISNULL(), NVL(), IFNULL() and
COALESCE() functions.
SQL NULL Functions
SQL ISNULL(), NVL(), IFNULL() and COALESCE() Functions
Look at the following "Products" table:
|
P_Id
|
ProductName
|
UnitPrice
|
UnitsInStock
|
UnitsOnOrder
|
|
1
|
Jarlsberg
|
10.45
|
16
|
15
|
|
2
|
Mascarpone
|
32.56
|
23
|
|
|
3
|
Gorgonzola
|
15.67
|
9
|
20
|
Suppose that the "UnitsOnOrder" column is optional, and may
contain NULL values.
We have the following SELECT statement:
|
SELECT
ProductName,UnitPrice*(UnitsInStock+UnitsOnOrder)
FROM Products
|
In the example above, if any of the "UnitsOnOrder" values are
NULL, the result is NULL.
Microsoft's ISNULL() function is used to specify how we want to treat NULL
values.
The NVL(), IFNULL(), and COALESCE() functions can also be used to achieve
the same result.
In this case we want NULL values to be zero.
Below, if "UnitsOnOrder" is NULL it will not harm the calculation,
because ISNULL() returns a zero if the value is NULL:
SQL Server / MS Access
|
SELECT
ProductName,UnitPrice*(UnitsInStock+ISNULL(UnitsOnOrder,0))
FROM Products
|
Oracle
Oracle does not have an ISNULL() function. However, we can use the NVL()
function to achieve the same result:
|
SELECT
ProductName,UnitPrice*(UnitsInStock+NVL(UnitsOnOrder,0))
FROM Products
|
MySQL
MySQL does have an ISNULL() function. However, it works a little bit
different from Microsoft's ISNULL() function.
In MySQL we can use the IFNULL() function, like this:
|
SELECT
ProductName,UnitPrice*(UnitsInStock+IFNULL(UnitsOnOrder,0))
FROM Products
|
or we can use the COALESCE() function, like this:
|
SELECT ProductName,UnitPrice*(UnitsInStock+COALESCE(UnitsOnOrder,0))
FROM Products
|
SQL Data Types
Data types and ranges for Microsoft Access, MySQL and SQL
Server.
Microsoft Access Data Types
|
Data type
|
Description
|
Storage
|
|
Text
|
Use for text or combinations
of text and numbers. 255 characters maximum
|
|
|
Memo
|
Memo is used for larger
amounts of text. Stores up to 65,536 characters. Note: You cannot sort
a memo field. However, they are searchable
|
|
|
Byte
|
Allows whole numbers from 0
to 255
|
1 byte
|
|
Integer
|
Allows whole numbers between
-32,768 and 32,767
|
2 bytes
|
|
Long
|
Allows whole numbers between
-2,147,483,648 and 2,147,483,647
|
4 bytes
|
|
Single
|
Single precision
floating-point. Will handle most decimals
|
4 bytes
|
|
Double
|
Double precision
floating-point. Will handle most decimals
|
8 bytes
|
|
Currency
|
Use for currency. Holds up
to 15 digits of whole dollars, plus 4 decimal places. Tip: You can
choose which country's currency to use
|
8 bytes
|
|
AutoNumber
|
AutoNumber fields
automatically give each record its own number, usually starting at 1
|
4 bytes
|
|
Date/Time
|
Use for dates and times
|
8 bytes
|
|
Yes/No
|
A logical field can be
displayed as Yes/No, True/False, or On/Off. In code, use the constants True
and False (equivalent to -1 and 0). Note: Null values are not allowed
in Yes/No fields
|
1 bit
|
|
Ole Object
|
Can store pictures, audio,
video, or other BLOBs (Binary Large OBjects)
|
up to 1GB
|
|
Hyperlink
|
Contain links to other
files, including web pages
|
|
|
Lookup Wizard
|
Let you type a list of
options, which can then be chosen from a drop-down list
|
4 bytes
|
MySQL Data Types
In MySQL there are three main types : text, number, and Date/Time types.
Text types:
|
Data type
|
Description
|
|
CHAR(size)
|
Holds a fixed length string
(can contain letters, numbers, and special characters). The fixed size is
specified in parenthesis. Can store up to 255 characters
|
|
VARCHAR(size)
|
Holds a variable length
string (can contain letters, numbers, and special characters). The maximum
size is specified in parenthesis. Can store up to 255 characters. Note:
If you put a greater value than 255 it will be converted to a TEXT type
|
|
TINYTEXT
|
Holds a string with a
maximum length of 255 characters
|
|
TEXT
|
Holds a string with a
maximum length of 65,535 characters
|
|
BLOB
|
For BLOBs (Binary Large
OBjects). Holds up to 65,535 bytes of data
|
|
MEDIUMTEXT
|
Holds a string with a
maximum length of 16,777,215 characters
|
|
MEDIUMBLOB
|
For BLOBs (Binary Large
OBjects). Holds up to 16,777,215 bytes of data
|
|
LONGTEXT
|
Holds a string with a
maximum length of 4,294,967,295 characters
|
|
LONGBLOB
|
For BLOBs (Binary Large
OBjects). Holds up to 4,294,967,295 bytes of data
|
|
ENUM(x,y,z,etc.)
|
Let you enter a list of possible
values. You can list up to 65535 values in an ENUM list. If a value is
inserted that is not in the list, a blank value will be inserted.
Note: The values are sorted in the order you enter them.
You enter the possible
values in this format: ENUM('X','Y','Z')
|
|
SET
|
Similar to ENUM except that
SET may contain up to 64 list items and can store more than one choice
|
Number types:
|
Data type
|
Description
|
|
TINYINT(size)
|
-128 to 127 normal. 0 to 255
UNSIGNED*. The maximum number of digits may be specified in parenthesis
|
|
SMALLINT(size)
|
-32768 to 32767 normal. 0 to
65535 UNSIGNED*. The maximum number of digits may be specified in parenthesis
|
|
MEDIUMINT(size)
|
-8388608 to 8388607 normal.
0 to 16777215 UNSIGNED*. The maximum number of digits may be specified in parenthesis
|
|
INT(size)
|
-2147483648 to 2147483647
normal. 0 to 4294967295 UNSIGNED*. The maximum number of digits may be
specified in parenthesis
|
|
BIGINT(size)
|
-9223372036854775808 to
9223372036854775807 normal. 0 to 18446744073709551615 UNSIGNED*. The maximum
number of digits may be specified in parenthesis
|
|
FLOAT(size,d)
|
A small number with a
floating decimal point. The maximum number of digits may be specified in the
size parameter. The maximum number of digits to the right of the decimal
point is specified in the d parameter
|
|
DOUBLE(size,d)
|
A large number with a
floating decimal point. The maximum number of digits may be specified in the
size parameter. The maximum number of digits to the right of the decimal
point is specified in the d parameter
|
|
DECIMAL(size,d)
|
A DOUBLE stored as a string
, allowing for a fixed decimal point. The maximum number of digits may be
specified in the size parameter. The maximum number of digits to the right of
the decimal point is specified in the d parameter
|
*The integer types have an extra option called UNSIGNED. Normally, the
integer goes from an negative to positive value. Adding the UNSIGNED attribute
will move that range up so it starts at zero instead of a negative number.
Date types:
|
Data type
|
Description
|
|
DATE()
|
A date. Format: YYYY-MM-DD
Note: The supported
range is from '1000-01-01' to '9999-12-31'
|
|
DATETIME()
|
*A date and time
combination. Format: YYYY-MM-DD HH:MM:SS
Note: The supported
range is from '1000-01-01 00:00:00' to '9999-12-31 23:59:59'
|
|
TIMESTAMP()
|
*A timestamp. TIMESTAMP
values are stored as the number of seconds since the Unix epoch ('1970-01-01
00:00:00' UTC). Format: YYYY-MM-DD HH:MM:SS
Note: The supported
range is from '1970-01-01 00:00:01' UTC to '2038-01-09 03:14:07' UTC
|
|
TIME()
|
A time. Format: HH:MM:SS
Note: The supported
range is from '-838:59:59' to '838:59:59'
|
|
YEAR()
|
A year in two-digit or
four-digit format.
Note: Values
allowed in four-digit format: 1901 to 2155. Values allowed in two-digit
format: 70 to 69, representing years from 1970 to 2069
|
*Even if DATETIME and TIMESTAMP return the same format, they work very
differently. In an INSERT or UPDATE query, the TIMESTAMP automatically set
itself to the current date and time. TIMESTAMP also accepts various formats,
like YYYYMMDDHHMMSS, YYMMDDHHMMSS, YYYYMMDD, or YYMMDD.
SQL Server Data Types
Character strings:
|
Data type
|
Description
|
Storage
|
|
char(n)
|
Fixed-length character
string. Maximum 8,000 characters
|
n
|
|
varchar(n)
|
Variable-length character
string. Maximum 8,000 characters
|
|
|
varchar(max)
|
Variable-length character
string. Maximum 1,073,741,824 characters
|
|
|
text
|
Variable-length character
string. Maximum 2GB of text data
|
|
Unicode strings:
|
Data type
|
Description
|
Storage
|
|
nchar(n)
|
Fixed-length Unicode data.
Maximum 4,000 characters
|
|
|
nvarchar(n)
|
Variable-length Unicode
data. Maximum 4,000 characters
|
|
|
nvarchar(max)
|
Variable-length Unicode
data. Maximum 536,870,912 characters
|
|
|
ntext
|
Variable-length Unicode
data. Maximum 2GB of text data
|
|
Binary types:
|
Data type
|
Description
|
Storage
|
|
bit
|
Allows 0, 1, or NULL
|
|
|
binary(n)
|
Fixed-length binary data.
Maximum 8,000 bytes
|
|
|
varbinary(n)
|
Variable-length binary data.
Maximum 8,000 bytes
|
|
|
varbinary(max)
|
Variable-length binary data.
Maximum 2GB
|
|
|
image
|
Variable-length binary data.
Maximum 2GB
|
|
Number types:
|
Data type
|
Description
|
Storage
|
|
tinyint
|
Allows whole numbers from 0
to 255
|
1 byte
|
|
smallint
|
Allows whole numbers between
-32,768 and 32,767
|
2 bytes
|
|
int
|
Allows whole numbers between
-2,147,483,648 and 2,147,483,647
|
4 bytes
|
|
bigint
|
Allows whole numbers between
-9,223,372,036,854,775,808 and 9,223,372,036,854,775,807
|
8 bytes
|
|
decimal(p,s)
|
Fixed precision and scale
numbers.
Allows numbers from -10^38 +1 to 10^38 –1.
The p parameter indicates the maximum total number of digits that can be
stored (both to the left and to the right of the decimal point). p must be a
value from 1 to 38. Default is 18.
The s parameter indicates
the maximum number of digits stored to the right of the decimal point. s must
be a value from 0 to p. Default value is 0
|
5-17 bytes
|
|
numeric(p,s)
|
Fixed precision and scale
numbers.
Allows numbers from -10^38 +1 to 10^38 –1.
The p parameter indicates the maximum total number of digits that can be
stored (both to the left and to the right of the decimal point). p must be a
value from 1 to 38. Default is 18.
The s parameter indicates
the maximum number of digits stored to the right of the decimal point. s must
be a value from 0 to p. Default value is 0
|
5-17 bytes
|
|
smallmoney
|
Monetary data from
-214,748.3648 to 214,748.3647
|
4 bytes
|
|
money
|
Monetary data from
-922,337,203,685,477.5808 to 922,337,203,685,477.5807
|
8 bytes
|
|
float(n)
|
Floating precision number
data from -1.79E + 308 to 1.79E + 308.
The n parameter indicates
whether the field should hold 4 or 8 bytes. float(24) holds a 4-byte field
and float(53) holds an 8-byte field. Default value of n is 53.
|
4 or 8 bytes
|
|
real
|
Floating precision number
data from -3.40E + 38 to 3.40E + 38
|
4 bytes
|
Date types:
|
Data type
|
Description
|
Storage
|
|
datetime
|
From January 1, 1753 to
December 31, 9999 with an accuracy of 3.33 milliseconds
|
8 bytes
|
|
datetime2
|
From January 1, 0001 to
December 31, 9999 with an accuracy of 100 nanoseconds
|
6-8 bytes
|
|
smalldatetime
|
From January 1, 1900 to June
6, 2079 with an accuracy of 1 minute
|
4 bytes
|
|
date
|
Store a date only. From
January 1, 0001 to December 31, 9999
|
3 bytes
|
|
time
|
Store a time only to an
accuracy of 100 nanoseconds
|
3-5 bytes
|
|
datetimeoffset
|
The same as datetime2 with
the addition of a time zone offset
|
8-10 bytes
|
|
timestamp
|
Stores a unique number that
gets updated every time a row gets created or modified. The timestamp value
is based upon an internal clock and does not correspond to real time. Each
table may have only one timestamp variable
|
|
Other data types:
|
Data type
|
Description
|
|
sql_variant
|
Stores up to 8,000 bytes of
data of various data types, except text, ntext, and timestamp
|
|
uniqueidentifier
|
Stores a globally unique
identifier (GUID)
|
|
xml
|
Stores XML formatted data.
Maximum 2GB
|
|
cursor
|
Stores a reference to a
cursor used for database operations
|
|
table
|
Stores a result-set for
later processing
|
SQL Functions
SQL has many built-in functions for performing calculations on
data.
SQL Aggregate Functions
SQL aggregate functions return a single value, calculated from values in a
column.
Useful aggregate functions:
·
AVG() - Returns the average value
·
COUNT() - Returns the number of rows
·
FIRST() - Returns the first value
·
LAST() - Returns the last value
·
MAX() - Returns the largest value
·
MIN() - Returns the smallest value
·
SUM() - Returns the sum
SQL Scalar functions
SQL scalar functions return a single value, based on the input value.
Useful scalar functions:
·
UCASE() - Converts a field to upper case
·
LCASE() - Converts a field to lower case
·
MID() - Extract characters from a text field
·
LEN() - Returns the length of a text field
·
ROUND() - Rounds a numeric field to the number
of decimals specified
·
NOW() - Returns the current system date and time
·
FORMAT() - Formats how a field is to be
displayed
Tip: The aggregate functions and the scalar functions will be
explained in details in the next chapters.
SQL AVG() Function
The AVG() Function
The AVG() function returns the average value of a numeric column.
SQL AVG() Syntax
|
SELECT AVG(column_name) FROM
table_name
|
SQL AVG() Example
We have the following "Orders" table:
|
O_Id
|
OrderDate
|
OrderPrice
|
Customer
|
|
1
|
2008/11/12
|
1000
|
Hansen
|
|
2
|
2008/10/23
|
1600
|
Nilsen
|
|
3
|
2008/09/02
|
700
|
Hansen
|
|
4
|
2008/09/03
|
300
|
Hansen
|
|
5
|
2008/08/30
|
2000
|
Jensen
|
|
6
|
2008/10/04
|
100
|
Nilsen
|
Now we want to find the average value of the "OrderPrice" fields.
We use the following SQL statement:
|
SELECT AVG(OrderPrice) AS
OrderAverage FROM Orders
|
The result-set will look like this:
Now we want to find the customers that have an OrderPrice value higher than
the average OrderPrice value.
We use the following SQL statement:
|
SELECT Customer FROM Orders
WHERE OrderPrice>(SELECT AVG(OrderPrice) FROM Orders)
|
The result-set will look like this:
|
Customer
|
|
Hansen
|
|
Nilsen
|
|
Jensen
|
SQL COUNT() Function
The COUNT() function returns the number of rows that matches a
specified criteria.
SQL COUNT(column_name) Syntax
The COUNT(column_name) function returns the number of values (NULL values
will not be counted) of the specified column:
|
SELECT COUNT(column_name)
FROM table_name
|
SQL COUNT(*) Syntax
The COUNT(*) function returns the number of records in a table:
|
SELECT COUNT(*) FROM
table_name
|
SQL COUNT(DISTINCT column_name) Syntax
The COUNT(DISTINCT column_name) function returns the number of distinct
values of the specified column:
|
SELECT COUNT(DISTINCT
column_name) FROM table_name
|
Note: COUNT(DISTINCT) works with ORACLE and Microsoft SQL Server, but
not with Microsoft Access.
SQL COUNT(column_name) Example
We have the following "Orders" table:
|
O_Id
|
OrderDate
|
OrderPrice
|
Customer
|
|
1
|
2008/11/12
|
1000
|
Hansen
|
|
2
|
2008/10/23
|
1600
|
Nilsen
|
|
3
|
2008/09/02
|
700
|
Hansen
|
|
4
|
2008/09/03
|
300
|
Hansen
|
|
5
|
2008/08/30
|
2000
|
Jensen
|
|
6
|
2008/10/04
|
100
|
Nilsen
|
Now we want to count the number of orders from "Customer Nilsen".
We use the following SQL statement:
|
SELECT COUNT(Customer) AS
CustomerNilsen FROM Orders
WHERE Customer='Nilsen'
|
The result of the SQL statement above will be 2, because the customer Nilsen
has made 2 orders in total:
SQL COUNT(*) Example
If we omit the WHERE clause, like this:
|
SELECT COUNT(*) AS
NumberOfOrders FROM Orders
|
The result-set will look like this:
which is the total number of rows in the table.
SQL COUNT(DISTINCT column_name) Example
Now we want to count the number of unique customers in the
"Orders" table.
We use the following SQL statement:
|
SELECT COUNT(DISTINCT
Customer) AS NumberOfCustomers FROM Orders
|
The result-set will look like this:
which is the number of unique customers (Hansen, Nilsen, and Jensen) in the
"Orders" table.
SQL FIRST() Function
The FIRST() Function
The FIRST() function returns the first value of the selected column.
SQL FIRST() Syntax
|
SELECT FIRST(column_name)
FROM table_name
|
SQL FIRST() Example
We have the following "Orders" table:
|
O_Id
|
OrderDate
|
OrderPrice
|
Customer
|
|
1
|
2008/11/12
|
1000
|
Hansen
|
|
2
|
2008/10/23
|
1600
|
Nilsen
|
|
3
|
2008/09/02
|
700
|
Hansen
|
|
4
|
2008/09/03
|
300
|
Hansen
|
|
5
|
2008/08/30
|
2000
|
Jensen
|
|
6
|
2008/10/04
|
100
|
Nilsen
|
Now we want to find the first value of the "OrderPrice" column.
We use the following SQL statement:
|
SELECT FIRST(OrderPrice) AS
FirstOrderPrice FROM Orders
|
Tip: Workaround if FIRST() function is
not supported:
|
SELECT OrderPrice FROM
Orders ORDER BY O_Id LIMIT 1
|
The result-set will look like this:
SQL LAST() Function
The LAST() Function
The LAST() function returns the last value of the selected column.
SQL LAST() Syntax
|
SELECT LAST(column_name)
FROM table_name
|
SQL LAST() Example
We have the following "Orders" table:
|
O_Id
|
OrderDate
|
OrderPrice
|
Customer
|
|
1
|
2008/11/12
|
1000
|
Hansen
|
|
2
|
2008/10/23
|
1600
|
Nilsen
|
|
3
|
2008/09/02
|
700
|
Hansen
|
|
4
|
2008/09/03
|
300
|
Hansen
|
|
5
|
2008/08/30
|
2000
|
Jensen
|
|
6
|
2008/10/04
|
100
|
Nilsen
|
Now we want to find the last value of the "OrderPrice" column.
We use the following SQL statement:
|
SELECT LAST(OrderPrice) AS
LastOrderPrice FROM Orders
|
Tip: Workaround if LAST() function is
not supported:
|
SELECT OrderPrice FROM
Orders ORDER BY O_Id DESC LIMIT 1
|
The result-set will look like this:
SQL MAX() Function
The MAX() Function
The MAX() function returns the largest value of the selected column.
SQL MAX() Syntax
|
SELECT MAX(column_name) FROM
table_name
|
SQL MAX() Example
We have the following "Orders" table:
|
O_Id
|
OrderDate
|
OrderPrice
|
Customer
|
|
1
|
2008/11/12
|
1000
|
Hansen
|
|
2
|
2008/10/23
|
1600
|
Nilsen
|
|
3
|
2008/09/02
|
700
|
Hansen
|
|
4
|
2008/09/03
|
300
|
Hansen
|
|
5
|
2008/08/30
|
2000
|
Jensen
|
|
6
|
2008/10/04
|
100
|
Nilsen
|
Now we want to find the largest value of the "OrderPrice" column.
We use the following SQL statement:
|
SELECT MAX(OrderPrice) AS
LargestOrderPrice FROM Orders
|
The result-set will look like this:
SQL MIN() Function
The MIN() Function
The MIN() function returns the smallest value of the selected column.
SQL MIN() Syntax
|
SELECT MIN(column_name) FROM
table_name
|
SQL MIN() Example
We have the following "Orders" table:
|
O_Id
|
OrderDate
|
OrderPrice
|
Customer
|
|
1
|
2008/11/12
|
1000
|
Hansen
|
|
2
|
2008/10/23
|
1600
|
Nilsen
|
|
3
|
2008/09/02
|
700
|
Hansen
|
|
4
|
2008/09/03
|
300
|
Hansen
|
|
5
|
2008/08/30
|
2000
|
Jensen
|
|
6
|
2008/10/04
|
100
|
Nilsen
|
Now we want to find the smallest value of the "OrderPrice" column.
We use the following SQL statement:
|
SELECT MIN(OrderPrice) AS
SmallestOrderPrice FROM Orders
|
The result-set will look like this:
SQL SUM() Function
The SUM() Function
The SUM() function returns the total sum of a numeric column.
SQL SUM() Syntax
|
SELECT SUM(column_name) FROM
table_name
|
SQL SUM() Example
We have the following "Orders" table:
|
O_Id
|
OrderDate
|
OrderPrice
|
Customer
|
|
1
|
2008/11/12
|
1000
|
Hansen
|
|
2
|
2008/10/23
|
1600
|
Nilsen
|
|
3
|
2008/09/02
|
700
|
Hansen
|
|
4
|
2008/09/03
|
300
|
Hansen
|
|
5
|
2008/08/30
|
2000
|
Jensen
|
|
6
|
2008/10/04
|
100
|
Nilsen
|
Now we want to find the sum of all "OrderPrice" fields".
We use the following SQL statement:
|
SELECT SUM(OrderPrice) AS
OrderTotal FROM Orders
|
The result-set will look like this:
SQL GROUP BY Statement
Aggregate functions often need an added GROUP BY statement.
The GROUP BY Statement
The GROUP BY statement is used in conjunction with the aggregate functions
to group the result-set by one or more columns.
SQL GROUP BY Syntax
|
SELECT column_name,
aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
|
SQL GROUP BY Example
We have the following "Orders" table:
|
O_Id
|
OrderDate
|
OrderPrice
|
Customer
|
|
1
|
2008/11/12
|
1000
|
Hansen
|
|
2
|
2008/10/23
|
1600
|
Nilsen
|
|
3
|
2008/09/02
|
700
|
Hansen
|
|
4
|
2008/09/03
|
300
|
Hansen
|
|
5
|
2008/08/30
|
2000
|
Jensen
|
|
6
|
2008/10/04
|
100
|
Nilsen
|
Now we want to find the total sum (total order) of each customer.
We will have to use the GROUP BY statement to group the customers.
We use the following SQL statement:
|
SELECT
Customer,SUM(OrderPrice) FROM Orders
GROUP BY Customer
|
The result-set will look like this:
|
Customer
|
SUM(OrderPrice)
|
|
Hansen
|
2000
|
|
Nilsen
|
1700
|
|
Jensen
|
2000
|
Nice! Isn't it? :)
Let's see what happens if we omit the GROUP BY statement:
|
SELECT
Customer,SUM(OrderPrice) FROM Orders
|
The result-set will look like this:
|
Customer
|
SUM(OrderPrice)
|
|
Hansen
|
5700
|
|
Nilsen
|
5700
|
|
Hansen
|
5700
|
|
Hansen
|
5700
|
|
Jensen
|
5700
|
|
Nilsen
|
5700
|
The result-set above is not what we wanted.
Explanation of why the above SELECT statement cannot be used: The
SELECT statement above has two columns specified (Customer and SUM(OrderPrice).
The "SUM(OrderPrice)" returns a single value (that is the total sum
of the "OrderPrice" column), while "Customer" returns 6
values (one value for each row in the "Orders" table). This will
therefore not give us the correct result. However, you have seen that the GROUP
BY statement solves this problem.
GROUP BY More Than One Column
We can also use the GROUP BY statement on more than one column, like this:
|
SELECT
Customer,OrderDate,SUM(OrderPrice) FROM Orders
GROUP BY Customer,OrderDate
|
SQL HAVING Clause
The HAVING Clause
The HAVING clause was added to SQL because the WHERE keyword could not be
used with aggregate functions.
SQL HAVING Syntax
|
SELECT column_name,
aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING aggregate_function(column_name) operator value
|
SQL HAVING Example
We have the following "Orders" table:
|
O_Id
|
OrderDate
|
OrderPrice
|
Customer
|
|
1
|
2008/11/12
|
1000
|
Hansen
|
|
2
|
2008/10/23
|
1600
|
Nilsen
|
|
3
|
2008/09/02
|
700
|
Hansen
|
|
4
|
2008/09/03
|
300
|
Hansen
|
|
5
|
2008/08/30
|
2000
|
Jensen
|
|
6
|
2008/10/04
|
100
|
Nilsen
|
Now we want to find if any of the customers have a total order of less than
2000.
We use the following SQL statement:
|
SELECT
Customer,SUM(OrderPrice) FROM Orders
GROUP BY Customer
HAVING SUM(OrderPrice)<2000
|
The result-set will look like this:
|
Customer
|
SUM(OrderPrice)
|
|
Nilsen
|
1700
|
Now we want to find if the customers "Hansen" or
"Jensen" have a total order of more than 1500.
We add an ordinary WHERE clause to the SQL statement:
|
SELECT
Customer,SUM(OrderPrice) FROM Orders
WHERE Customer='Hansen' OR Customer='Jensen'
GROUP BY Customer
HAVING SUM(OrderPrice)>1500
|
The result-set will look like this:
|
Customer
|
SUM(OrderPrice)
|
|
Hansen
|
2000
|
|
Jensen
|
2000
|
SQL UCASE() Function
The UCASE() Function
The UCASE() function converts the value of a field to uppercase.
SQL UCASE() Syntax
|
SELECT UCASE(column_name)
FROM table_name
|
Syntax for SQL Server
|
SELECT UPPER(column_name)
FROM table_name
|
SQL UCASE() Example
We have the following "Persons" table:
|
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
Now we want to select the content of the "LastName" and
"FirstName" columns above, and convert the "LastName"
column to uppercase.
We use the following SELECT statement:
|
SELECT UCASE(LastName) as
LastName,FirstName FROM Persons
|
The result-set will look like this:
|
LastName
|
FirstName
|
|
HANSEN
|
Ola
|
|
SVENDSON
|
Tove
|
|
PETTERSEN
|
Kari
|
SQL LCASE() Function
The LCASE() Function
The LCASE() function converts the value of a field to lowercase.
SQL LCASE() Syntax
|
SELECT LCASE(column_name)
FROM table_name
|
Syntax for SQL Server
|
SELECT LOWER(column_name)
FROM table_name
|
SQL LCASE() Example
We have the following "Persons" table:
|
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
Now we want to select the content of the "LastName" and
"FirstName" columns above, and convert the "LastName"
column to lowercase.
We use the following SELECT statement:
|
SELECT LCASE(LastName) as
LastName,FirstName FROM Persons
|
The result-set will look like this:
|
LastName
|
FirstName
|
|
hansen
|
Ola
|
|
svendson
|
Tove
|
|
pettersen
|
Kari
|
SQL MID() Function
The MID() Function
The MID() function is used to extract characters from a text field.
SQL MID() Syntax
|
SELECT
MID(column_name,start[,length]) FROM table_name
|
|
Parameter
|
Description
|
|
column_name
|
Required. The field to
extract characters from
|
|
start
|
Required. Specifies the
starting position (starts at 1)
|
|
length
|
Optional. The number of
characters to return. If omitted, the MID() function returns the rest of the
text
|
SQL MID() Example
We have the following "Persons" table:
|
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
Now we want to extract the first four characters of the "City"
column above.
We use the following SELECT statement:
|
SELECT MID(City,1,4) as
SmallCity FROM Persons
|
The result-set will look like this:
SQL LEN() Function
The LEN() Function
The LEN() function returns the length of the value in a text field.
SQL LEN() Syntax
|
SELECT LEN(column_name) FROM
table_name
|
SQL LEN() Example
We have the following "Persons" table:
|
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
Now we want to select the length of the values in the "Address"
column above.
We use the following SELECT statement:
|
SELECT LEN(Address) as
LengthOfAddress FROM Persons
|
The result-set will look like this:
SQL ROUND() Function
The ROUND() Function
The ROUND() function is used to round a numeric field to the number of
decimals specified.
SQL ROUND() Syntax
|
SELECT ROUND(column_name,decimals)
FROM table_name
|
|
Parameter
|
Description
|
|
column_name
|
Required. The field to
round.
|
|
decimals
|
Required. Specifies the
number of decimals to be returned.
|
SQL ROUND() Example
We have the following "Products" table:
|
Prod_Id
|
ProductName
|
Unit
|
UnitPrice
|
|
1
|
Jarlsberg
|
1000 g
|
10.45
|
|
2
|
Mascarpone
|
1000 g
|
32.56
|
|
3
|
Gorgonzola
|
1000 g
|
15.67
|
Now we want to display the product name and the price rounded to the nearest
integer.
We use the following SELECT statement:
|
SELECT ProductName,
ROUND(UnitPrice,0) as UnitPrice FROM Products
|
The result-set will look like this:
|
ProductName
|
UnitPrice
|
|
Jarlsberg
|
10
|
|
Mascarpone
|
33
|
|
Gorgonzola
|
16
|
SQL NOW() Function
The NOW() Function
The NOW() function returns the current system date and time.
SQL NOW() Syntax
|
SELECT NOW() FROM table_name
|
SQL NOW() Example
We have the following "Products" table:
|
Prod_Id
|
ProductName
|
Unit
|
UnitPrice
|
|
1
|
Jarlsberg
|
1000 g
|
10.45
|
|
2
|
Mascarpone
|
1000 g
|
32.56
|
|
3
|
Gorgonzola
|
1000 g
|
15.67
|
Now we want to display the products and prices per today's date.
We use the following SELECT statement:
|
SELECT ProductName,
UnitPrice, Now() as PerDate FROM Products
|
The result-set will look like this:
|
ProductName
|
UnitPrice
|
PerDate
|
|
Jarlsberg
|
10.45
|
10/7/2008 11:25:02 AM
|
|
Mascarpone
|
32.56
|
10/7/2008 11:25:02 AM
|
|
Gorgonzola
|
15.67
|
10/7/2008 11:25:02 AM
|
SQL FORMAT() Function
The FORMAT() Function
The FORMAT() function is used to format how a field is to be displayed.
SQL FORMAT() Syntax
|
SELECT
FORMAT(column_name,format) FROM table_name
|
|
Parameter
|
Description
|
|
column_name
|
Required. The field to be
formatted.
|
|
format
|
Required. Specifies the
format.
|
SQL FORMAT() Example
We have the following "Products" table:
|
Prod_Id
|
ProductName
|
Unit
|
UnitPrice
|
|
1
|
Jarlsberg
|
1000 g
|
10.45
|
|
2
|
Mascarpone
|
1000 g
|
32.56
|
|
3
|
Gorgonzola
|
1000 g
|
15.67
|
Now we want to display the products and prices per today's date (with
today's date displayed in the following format "YYYY-MM-DD").
We use the following SELECT statement:
|
SELECT ProductName,
UnitPrice, FORMAT(Now(),'YYYY-MM-DD') as PerDate
FROM Products
|
The result-set will look like this:
|
ProductName
|
UnitPrice
|
PerDate
|
|
Jarlsberg
|
10.45
|
2008-10-07
|
|
Mascarpone
|
32.56
|
2008-10-07
|
|
Gorgonzola
|
15.67
|
2008-10-07
|
