In its simplest form, a foreign key is a referential constraint between two tables. A foreign key constraint validates the values of an INSERT or UPDATE against the values in another column, either in a different table or another column in the same table. Generally, though, a foreign key is a field (or fields) that points to the primary key of another table.
A foreign key can also be described as a column in a table that does NOT uniquely identify rows in that table, but is used as a link to matching columns in other tables to indicate a relationship.
A foreign key always defines a parent/child relationship. The "parent" is the column that is referenced in the foreign key and the "child" is the column or columns that contain the foreign key constraint.
In this example, the Customer_SID column in the ORDERS table is a foreign key pointing to the SID column in the CUSTOMER table:
CREATE TABLE ORDERS
(Order_ID integer primary key,
Customer_SID integer references CUSTOMER(SID),
You may also want to see additional information in the entry on the CONSTRAINT clause.
Related Code Snippets: