A foreign key is a key used to link two tables together. This is sometimes called a referencing key.
Foreign Key is a column or a combination of columns whose values match a Primary Key in a different table.
The relationship between 2 tables matches the Primary Key in one of the tables with a Foreign Key in the second table.
If a table has a primary key defined on any field(s), then you can not have two records having the same value of that field(s).
Consider the structure of the two tables as follows:
CREATE TABLE CUSTOMERS( ID INT NOT NULL, NAME VARCHAR (20) NOT NULL, AGE INT NOT NULL, ADDRESS CHAR (25) , SALARY DECIMAL (18, 2), PRIMARY KEY (ID) );
CREATE TABLE ORDERS ( ID INT NOT NULL, DATE DATETIME, CUSTOMER_ID INT references CUSTOMERS(ID), AMOUNT double, PRIMARY KEY (ID) );
If ORDERS table has already been created, and the foreign key has not yet been set, use the syntax for specifying a foreign key by altering a table.
ALTER TABLE ORDERS ADD FOREIGN KEY (Customer_ID) REFERENCES CUSTOMERS (ID);
DROP a FOREIGN KEY Constraint:
To drop a FOREIGN KEY constraint, use the following SQL:
ALTER TABLE ORDERS DROP FOREIGN KEY;