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The CHECK constraint specifies that values for the column in INSERT or UPDATE statements must return TRUE or NULL for a Boolean expression. If any values return FALSE, the entire statement is rejected.
Details
- If you add a
CHECKconstraint to an existing table, added values, along with any updates to current values, are checked. To check the existing rows, useVALIDATE CONSTRAINT. CHECKconstraints may be specified at the column or table level and can reference other columns within the table. Internally, all column-levelCHECKconstraints are converted to table-level constraints so they can be handled consistently.- You can have multiple
CHECKconstraints on a single column but ideally, for performance optimization, these should be combined using the logical operators. For example:
warranty_period INT CHECK (warranty_period >= 0) CHECK (warranty_period <= 24) should be specified as:
warranty_period INT CHECK (warranty_period BETWEEN 0 AND 24) - When a column with a
CHECKconstraint is dropped, theCHECKconstraint is also dropped.
Syntax
CHECK constraints can be defined at the table level. However, if you only want the constraint to apply to a single column, it can be applied at the column level.
Column level
| Parameter | Description |
|---|---|
table_name | The name of the table you're creating. |
column_name | The name of the constrained column. |
column_type | The constrained column's data type. |
check_expr | An expression that returns a Boolean value; if the expression evaluates to FALSE, the value cannot be inserted. |
column_constraints | Any other column-level constraints you want to apply to this column. |
column_def | Definitions for any other columns in the table. |
table_constraints | Any table-level constraints you want to apply. |
Example
> CREATE TABLE inventories ( product_id INT NOT NULL, warehouse_id INT NOT NULL, quantity_on_hand INT NOT NULL CHECK (quantity_on_hand > 0), PRIMARY KEY (product_id, warehouse_id) ); Table level
| Parameter | Description |
|---|---|
table_name | The name of the table you're creating. |
column_def | Definitions for any other columns in the table. |
name | The name you want to use for the constraint, which must be unique to its table and follow these identifier rules. |
check_expr | An expression that returns a Boolean value; if the expression evaluates to FALSE, the value cannot be inserted. |
table_constraints | Any other table-level constraints you want to apply. |
Example
> CREATE TABLE inventories ( product_id INT NOT NULL, warehouse_id INT NOT NULL, quantity_on_hand INT NOT NULL, PRIMARY KEY (product_id, warehouse_id), CONSTRAINT ok_to_supply CHECK (quantity_on_hand > 0 AND warehouse_id BETWEEN 100 AND 200) ); Usage example
CHECK constraints may be specified at the column or table level and can reference other columns within the table. Internally, all column-level CHECK constraints are converted to table-level constraints so they can be handled in a consistent fashion.
> CREATE TABLE inventories ( product_id INT NOT NULL, warehouse_id INT NOT NULL, quantity_on_hand INT NOT NULL CHECK (quantity_on_hand > 0), PRIMARY KEY (product_id, warehouse_id) ); > INSERT INTO inventories (product_id, warehouse_id, quantity_on_hand) VALUES (1, 2, 0); pq: failed to satisfy CHECK constraint (quantity_on_hand > 0)