Foreign Key Actions

With the sub-clauses ON UPDATE and ON DELETE it is possible to specify an action to be taken on the affected foreign key column(s) when referenced values in the master table are changed:

NO ACTION: (the default) — Nothing is done
CASCADE: The change in the master table is propagated to the corresponding row(s) in the child table.If a key value changes, the corresponding key in the child records changes to the new value;if the master row is deleted, the child records are deleted.
SET DEFAULT: The foreign key columns in the affected rows will be set to their default values as they were when the foreign key constraint was defined.
SET NULL: The foreign key columns in the affected rows will be set to NULL.

The specified action, or the default NO ACTION, could cause a foreign key column to become invalid.For example, it could get a value that is not present in the master table.Such condition will cause the operation on the master table to fail with an error message.

Example

...
  CONSTRAINT FK_ORDERS_CUST
    FOREIGN KEY (CUSTOMER) REFERENCES CUSTOMERS (ID)
      ON UPDATE CASCADE ON DELETE SET NULL

`CHECK` Constraint

The CHECK constraint defines the condition the values inserted in this column or row must satisfy.A condition is a logical expression (also called a predicate) that can return the TRUE, FALSE and UNKNOWN values.A condition is considered satisfied if the predicate returns TRUE or value UNKNOWN (equivalent to NULL).If the predicate returns FALSE, the value will not be accepted.This condition is used for inserting a new row into the table (the INSERT statement) and for updating the existing value of the table column (the UPDATE statement) and also for statements where one of these actions may take place (UPDATE OR INSERT, MERGE).

Important

A CHECK constraint on a domain-based column does not replace an existing CHECK condition on the domain, but becomes an addition to it.The Firebird engine has no way, during definition, to verify that the extra CHECK does not conflict with the existing one.

CHECK constraints — whether defined at table level or column level — refer to table columns by their names.The use of the keyword VALUE as a placeholder — as in domain CHECK constraints — is not valid in the context of defining constraints in a table.

Example

with two column-level constraints and one at table-level:

CREATE TABLE PLACES (
  ...
  LAT DECIMAL(9, 6) CHECK (ABS(LAT) <=  90),
  LON DECIMAL(9, 6) CHECK (ABS(LON) <= 180),
  ...
  CONSTRAINT CHK_POLES CHECK (ABS(LAT) < 90 OR LON = 0)
);

Foreign Key Actions

Foreign Key Actions

CHECK Constraint

`CHECK` Constraint