The CHECK keyword is a constraint that limits the value that can be placed in a column.

The following creates a CHECK constraint on the user_age column when the Users table is created. The CHECK constraint ensures that you can not have any person less than 18 years of age.

CREATE TABLE Users (
    user_age int,
    CHECK (user_age >= 18)
);

SQL Server:

CREATE TABLE Users (
    user_age int CHECK (user_age >= 18)
);

The following creates a CHECK constraint on the user_age column on an existing table. In the SQL Server version, the constraint is named check_age.

MySQL:

ALTER TABLE Users
ADD CHECK (user_age >= 18);

SQL Server:

ALTER TABLE Users
ADD CONSTRAINT check_age CHECK (user_age >= 18);

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