The format is:

          [CONSTRAINT table_constraint_name]
            CHECK (condition)

The CHECK can be defined as a column constraint:

          CREATE TABLE      EMPLOYEE_TBL (
                 ...
                 SEX        CHAR(1)   CHECK (IN ('M', 'F')),
                 ... );

The CHECK can be defined as a named table constraint:

          CREATE TABLE EMPLOYEE_TBL (
                 ...
                 SEX        CHAR(1)    NOT NULL,
                 ...
                 CONSTRAINT SEX_CHK
                      CHECK (SEX IN ('M', 'F')),
                 ... );

                                            or

          CREATE TABLE EMPLOYEE_TBL (
                 ...
                 BADGE      CHAR(4) NOT NULL,
                 ...
                 CONSTRAINT BADGE_CHK
                      CHECK (
                      (SUBSTRING(BADGE FROM 1 FOR 1) = 'E')
                      AND
                      (CAST(SUBSTRING(BADGE FROM 2 FOR 3)
                      AS INTEGER(BETWEEN 0 AND 999))
                 ...        );

> If the CHECK condition is violated, the INSERT or UPDATE will not happen and an error condition will be returned.

> The SQL standard says that CHECK can not be used with CURRENT_DATETIME and CURRENT_USER.
Access, Postgre SQL, and SQL Server can CHECK these values.

> MySQL only handles CHECK constraints, but it can not handle named CHECK constraints.