CHECK Constraint

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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 CHECK constraint to an existing table, added values, along with any updates to current values, are checked. To check the existing rows, use VALIDATE CONSTRAINT.
• 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 consistently.
• You can have multiple CHECK constraints 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 CHECK constraint is dropped, the CHECK constraint 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

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

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)

See also

• Constraints
• DROP CONSTRAINT
• DEFAULT constraint
• REFERENCES constraint (Foreign Key)
• NOT NULL constraint
• PRIMARY KEY constraint
• UNIQUE constraint
• SHOW CONSTRAINTS

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