13.1.20.7 CHECK Constraints

Prior to MySQL 8.0.16, CREATE TABLE permits only the following limited version of table CHECK constraint syntax, which is parsed and ignored:

As of MySQL 8.0.16, CREATE TABLE permits the core features of table and column CHECK constraints, for all storage engines. CREATE TABLE permits the following CHECK constraint syntax, for both table constraints and column constraints:

The optional symbol specifies a name for the constraint. If omitted, MySQL generates a name from the table name, a literal _chk_, and an ordinal number (1, 2, 3, ...). Constraint names have a maximum length of 64 characters. They are case sensitive, but not accent sensitive.

expr specifies the constraint condition as a boolean expression that must evaluate to TRUE or UNKNOWN (for NULL values) for each row of the table. If the condition evaluates to FALSE, it fails and a constraint violation occurs. The effect of a violation depends on the statement being executed, as described later in this section.

The optional enforcement clause indicates whether the constraint is enforced:

A CHECK constraint is specified as either a table constraint or column constraint:

Consider this table definition:

The definition includes table constraints and column constraints, in named and unnamed formats:

As mentioned, MySQL generates a name for any CHECK constraint specified without one. To see the names generated for the preceding table definition, use SHOW CREATE TABLE:

The SQL standard specifies that all types of constraints (primary key, unique index, foreign key, check) belong to the same namespace. In MySQL, each constraint type has its own namespace per schema (database). Consequently, CHECK constraint names must be unique per schema; no two tables in the same schema can share a CHECK constraint name. (Exception: A TEMPORARY table hides a non-TEMPORARY table of the same name, so it can have the same CHECK constraint names as well.)

Beginning generated constraint names with the table name helps ensure schema uniqueness because table names also must be unique within the schema.

CHECK condition expressions must adhere to the following rules. An error occurs if an expression contains disallowed constructs.

Foreign key referential actions (ON UPDATE, ON DELETE) are prohibited on columns used in CHECK constraints. Likewise, CHECK constraints are prohibited on columns used in foreign key referential actions.

CHECK constraints are evaluated for INSERT, UPDATE, REPLACE, LOAD DATA, and LOAD XML statements and an error occurs if a constraint evaluates to FALSE. If an error occurs, handling of changes already applied differs for transactional and nontransactional storage engines, and also depends on whether strict SQL mode is in effect, as described in Strict SQL Mode.

CHECK constraints are evaluated for INSERT IGNORE, UPDATE IGNORE, LOAD DATA ... IGNORE, and LOAD XML ... IGNORE statements and a warning occurs if a constraint evaluates to FALSE. The insert or update for any offending row is skipped.

If the constraint expression evaluates to a data type that differs from the declared column type, implicit coercion to the declared type occurs according to the usual MySQL type-conversion rules. See Section 12.2, “Type Conversion in Expression Evaluation”. If type conversion fails or results in a loss of precision, an error occurs.