SQL LECTURE 13

Lecture 13

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.

MySQL:

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

MySQL:

ALTER TABLE Persons

DROP CHECK chk_Person

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 / MS Access:

ALTER TABLE Persons

ALTER COLUMN City SET DEFAULT 'SANDNES'

Oracle:

ALTER TABLE Persons

MODIFY City 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