CREATE TABLE permits the core
features of table and column
constraints, for all storage engines.
CREATE TABLE permits the
CHECK constraint syntax, for both
table constraints and column constraints:
[CONSTRAINT [symbol]] CHECK (expr) [[NOT] ENFORCED]
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
expr specifies the constraint
condition as a boolean expression that must evaluate to
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
The optional enforcement clause indicates whether the constraint is enforced:
If omitted or specified as
ENFORCED, the constraint is created and enforced.
If specified as
NOT ENFORCED, the constraint is created but not enforced.
CHECK constraint is specified as either a
table constraint or column constraint:
A table constraint does not appear within a column definition and can refer to any table column or columns. Forward references are permitted to columns appearing later in the table definition.
A column constraint appears within a column definition and can refer only to that column.
Consider this table definition:
CREATE TABLE t1 ( CHECK (c1 <> c2), c1 INT CHECK (c1 > 10), c2 INT CONSTRAINT c2_positive CHECK (c2 > 0), c3 INT CHECK (c3 < 100), CONSTRAINT c1_nonzero CHECK (c1 <> 0), CHECK (c1 > c3) );
The definition includes table constraints and column constraints, in named and unnamed formats:
The first constraint is a table constraint: It occurs outside any column definition, so it can (and does) refer to multiple table columns. This constraint contains forward references to columns not defined yet. No constraint name is specified, so MySQL generates a name.
The next three constraints are column constraints: Each occurs within a column definition, and thus can refer only to the column being defined. One of the constraints is named explicitly. MySQL generates a name for each of the other two.
The last two constraints are table constraints. One of them is named explicitly. MySQL generates a name for the other one.
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:
mysql> SHOW CREATE TABLE t1\G *************************** 1. row *************************** Table: t1 Create Table: CREATE TABLE `t1` ( `c1` int(11) DEFAULT NULL, `c2` int(11) DEFAULT NULL, `c3` int(11) DEFAULT NULL, CONSTRAINT `c1_nonzero` CHECK ((`c1` <> 0)), CONSTRAINT `c2_positive` CHECK ((`c2` > 0)), CONSTRAINT `t1_chk_1` CHECK ((`c1` <> `c2`)), CONSTRAINT `t1_chk_2` CHECK ((`c1` > 10)), CONSTRAINT `t1_chk_3` CHECK ((`c3` < 100)), CONSTRAINT `t1_chk_4` CHECK ((`c1` > `c3`)) ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
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
TEMPORARY table of the same name, so it
can have the same
CHECK constraint names as
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
Nongenerated and generated columns are permitted, except columns with the
AUTO_INCREMENTattribute and columns in other tables.
Literals, deterministic built-in functions, and operators are permitted. A function is deterministic if, given the same data in tables, multiple invocations produce the same result, independently of the connected user. Examples of functions that are nondeterministic and fail this definition:
Stored functions and loadable functions are not permitted.
Stored procedure and function parameters are not permitted.
Variables (system variables, user-defined variables, and stored program local variables) are not permitted.
Subqueries are not permitted.
Foreign key referential actions (
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
LOAD DATA, and
LOAD XML statements and an error
occurs if a constraint evaluates to
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
LOAD DATA ...
LOAD XML ...
IGNORE statements and a warning occurs if a constraint
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 12.3, “Type Conversion in Expression Evaluation”. If type conversion fails or results in a loss of precision, an error occurs.
Constraint expression evaluation uses the SQL mode in effect at evaluation time. If any component of the expression depends on the SQL mode, different results may occur for different uses of the table unless the SQL mode is the same during all uses.
The Information Schema
CHECK_CONSTRAINTS table provides
CHECK constraints defined
on tables. See
Section 26.3.5, “The INFORMATION_SCHEMA CHECK_CONSTRAINTS Table”.