FOREIGN KEY constraint to remove the inconsistency in two tables when data in one table depends on data in another table.
A FOREIGN KEY constraint associates one or more columns of a table (the foreign key) with an identical set of columns on which a PRIMARY KEY constraint has been defined (a primary key column in another table).
CREATE TABLE table_name (column_name data_type REFERENCES table_name (emp_id))
CONSTRAINT constraint_name FOREIGN KEY (column_name,..) REFERENCE table_name (column_name,..)
CREATE TABLE employee (emp_name char(4), emp_id varchar (16) REFERENCES engineer (emp_id))
The above command creates a FOREIGN KEY on the attribute emp_id of the employee table that reference the primary key emp_id of the engineer table. This will ensure that the employeeID code that is inserted into the employee table is checked against the engineer table for the validity.
If the employee table is exist and does not have foreign key defined, then table also modified using ALTER TABLE command.
ALTER TABLE employee ADD CONSTRAINT fkemp_id FOREIGN KEY (emp_id) REFERENCES engineer (emp_id)