foreign key
A type of pointer relationship, between rows in separate InnoDB tables. The foreign key relationship is defined on one column in both the parent table and the child table.
In addition to enabling fast lookup of related information, foreign keys help to enforce referential integrity, by preventing any of these pointers from becoming invalid as data is inserted, updated, and deleted. This enforcement mechanism is a type of constraint. A row that points to another table cannot be inserted if the associated foreign key value does not exist in the other table. If a row is deleted or its foreign key value changed, and rows in another table point to that foreign key value, the foreign key can be set up to prevent the deletion, cause the corresponding column values in the other table to become null, or automatically delete the corresponding rows in the other table.
One of the stages in designing a normalized database is to identify data that is duplicated, separate that data into a new table, and set up a foreign key relationship so that the multiple tables can be queried like a single table, using a join operation.
See also child table.
See also FOREIGN KEY constraint.
See also join.
See also normalized.
See also NULL.
See also parent table.
See also referential integrity.
See also relational.