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Primary Key and Foreign Key in MySQL

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Primary Key:

Primary Key is something which will uniquely identifies each record in a database table.



Primary key is always a unique value and cannot be null.

Each table should have a primary key, and each table can have only ONE primary key.


For example:

CREATE TABLE tbl_employees
(
eid int NOT NULL,
name varchar(255) NOT NULL,
address varchar(255),
email varchar(255),
mobile varchar(255),
PRIMARY KEY (eid)
)



Foreign Key:

Foreign key is a constraint in some table and it points to a primary key of another table.

For example:

We have a table called tbl_employee like below:
eid name address email mobile
1 Ryan ktm ryan@gmail.com 9874512663


In above table eid is a primary key.

And we have another table called tbl_salary like below:
sid eid salary
1 1 20000

In above table, sid is a primary key of tbl_salary and eid is the foreign key taken from the table "tbl_employee".




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