Saturday, 23 June 2018

FOREIGN KEY

A FOREIGN KEY in one table points to PRIMARY KEY in another table.

A foreign key can have a different name than the primary key it comes from.

The primary key used by a foreign key is also known as a parent key. The table where the primary key is from is known as a parent table.

The foreign key can be used to make sure that the row in one table have corresponding row in another table.

Foreign key value can be null, even though primary key value can't.

Foreign key don't have to be unique in fact, they often aren't.

Create table from use a FOREIGN KEY:-

CREATE TABLE department
(
D_id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
D_Name varchar (40),
E_id int,'
CONSTRAINT employee_Eid_fk
FOREIGN KEY (E_id) REFERENCES employee (E_id)



The CONSTRAINT clause allows to define constraints name for the foreign key constraint. If we omit it MySQL will generate a name automatically. It is optional.

The REFERENCES clause specifies the parent table and its columns to which the columns in the child table refer. The number of columns in the child table and parent table specified in the FOREIGN KEY and REFERENCES must be the same.



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