A foreign key is a key used to link two tables together. This is sometimes also called as a referencing key.
A 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 cannot have two records having the same value of that field(s).
Consider the structure of the following two tables.
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 the ORDERS table has already been created and the foreign key has not yet been set, the use the syntax for specifying a foreign key by altering a table.
ALTER TABLE ORDERS ADD FOREIGN KEY (Customer_ID) REFERENCES CUSTOMERS (ID);
To drop a FOREIGN KEY constraint, use the following SQL syntax.
ALTER TABLE ORDERS DROP FOREIGN KEY;