SQL CHECK 约束
SQL CHECK 约束
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 column it will allow 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 on CREATE TABLE
The following SQL creates a CHECK constraint on the "Age" column when the "Persons" table is created. The CHECK constraint ensures that the age of a person must be 18, or older
MySQL
CREATE TABLE Persons (
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
CHECK (Age>=18)
);SQL Server / Oracle / MS Access
CREATE TABLE Persons (
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int CHECK (Age>=18)
);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 (
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
City varchar(255),
CONSTRAINT CHK_Person CHECK (Age>=18 AND City='Sandnes')
);SQL CHECK on ALTER TABLE
To create a CHECK constraint on the "Age" column when the table is already created, use the following SQL
MySQL / SQL Server / Oracle / MS Access
ALTER TABLE Persons
ADD CHECK (Age>=18);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_PersonAge CHECK (Age>=18 AND City='Sandnes');DROP a CHECK Constraint
To drop a CHECK constraint, use the following SQL
SQL Server / Oracle / MS Access
ALTER TABLE Persons
DROP CONSTRAINT CHK_PersonAge;MySQL
ALTER TABLE Persons
DROP CHECK CHK_PersonAge;
