• Home
  • Jobs
  • Courses
  • Certifications
  • Companies
  • Online IDE
  • Login
  • Signup
MySQL
  • Introduction To MySQL
  • MySQL RDBMS
  • MySQL SQL
  • MySQL SELECT Statement
  • MySQL WHERE Clause
  • MySQL AND, OR And NOT Operators
  • MySQL ORDER BY Keyword
  • MySQL INSERT INTO Statement
  • MySQL NULL Values
  • MySQL UPDATE Statement
  • MySQL DELETE Statement
  • MySQL LIMIT Clause
  • MySQL MIN() And MAX() Functions
  • MySQL COUNT(), AVG() And SUM() Functions
  • MySQL LIKE Operator
  • MySQL Wildcards
  • MySQL IN Operator
  • MySQL BETWEEN
  • MySQL Aliases
  • MySQL Joins
  • MySQL INNER JOIN Keyword
  • MySQL LEFT JOIN Keyword
  • MySQL RIGHT JOIN Keyword
  • MySQL CROSS JOIN Keyword
  • MySQL Self Join
  • MySQL UNION Operator
  • MySQL GROUP BY Statement
  • MySQL HAVING Clause
  • MySQL EXISTS Operator
  • MySQL ANY And ALL Operators
  • MySQL INSERT INTO SELECT Statement
  • MySQL CASE Statement
  • MySQL NULL Functions
  • MySQL Comments
  • MySQL Operators
  • MySQL CREATE DATABASE Statement
  • MySQL DROP DATABASE Statement
  • MySQL CREATE TABLE Statement
  • MySQL DROP TABLE Statement
  • MySQL ALTER TABLE Statement
  • MySQL Constraints
  • MySQL NOT NULL Constraint
  • MySQL UNIQUE Constraint
  • MySQL PRIMARY KEY Constraint
  • MySQL FOREIGN KEY Constraint
  • MySQL CHECK Constraint
  • MySQL DEFAULT Constraint
  • MySQL CREATE INDEX Statement
  • MySQL AUTO INCREMENT Field
  • MySQL Working With Dates
  • MySQL Views
  • MySQL Data Types
  • MySQL Functions
  • Home
  • Courses
  • MySQL
  • MySQL FOREIGN KEY Constraint

MySQL FOREIGN KEY Constraint

Previous Next

MySQL FOREIGN KEY Constraint


The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables.

A FOREIGN KEY is a field (or collection of fields) in one table, that refers to the PRIMARY KEY in another table.

The table with the foreign key is called the child table, and the table with the primary key is called the referenced or parent table.

Look at the following two tables:

PersonID LastName FirstName Age
1 Hansen Ola 30
2 Svendson Tove 23
3 Pettersen Kari 20

Orders Table
OrderID OrderNumber PersonID
1 77895 3
2 44678 3
3 22456 2
4 24562 1


Notice that the "PersonID" column in the "Orders" table points to the "PersonID" column in the "Persons" table.

The "PersonID" column in the "Persons" table is the PRIMARY KEY in the "Persons" table.

The "PersonID" column in the "Orders" table is a FOREIGN KEY in the "Orders" table.

The FOREIGN KEY constraint prevents invalid data from being inserted into the foreign key column, because it has to be one of the values contained in the parent table.

 



Practice Excercise Practice now

FOREIGN KEY On CREATE TABLE

The following SQL creates a FOREIGN KEY on the "PersonID" column when the "Orders" table is created:
 

CREATE TABLE Orders (
    OrderID int NOT NULL,
    OrderNumber int NOT NULL,
    PersonID int,
    PRIMARY KEY (OrderID),
    FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
);
To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:
 
CREATE TABLE Orders (
    OrderID int NOT NULL,
    OrderNumber int NOT NULL,
    PersonID int,
    PRIMARY KEY (OrderID),
    CONSTRAINT FK_PersonOrder FOREIGN KEY (PersonID)
    REFERENCES Persons(PersonID)
);



Practice Excercise Practice now

FOREIGN KEY On ALTER TABLE

To create a FOREIGN KEY constraint on the "PersonID" column when the "Orders" table is already created, use the following SQL:
 

ALTER TABLE Orders
ADD FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);

To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:
 
ALTER TABLE Orders
ADD CONSTRAINT FK_PersonOrder
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);



Practice Excercise Practice now

DROP A FOREIGN KEY Constraint

To drop a FOREIGN KEY constraint, use the following SQL:
 

ALTER TABLE Orders
DROP FOREIGN KEY FK_PersonOrder;



Practice Excercise Practice now

Previous Next
COMPANY
  • About us
  • Careers
  • Contact Us
  • In Press
  • People
  • Companies List
Products
  • Features
  • Coding Assessments
  • Psychometric Assessment
  • Aptitude Assessments
  • Tech/Functional Assessments
  • Video Assessment
  • Fluency Assessment
  • Campus
 
  • Learning
  • Campus Recruitment
  • Lateral Recruitment
  • Enterprise
  • Education
  • K 12
  • Government
OTHERS
  • Blog
  • Terms of Services
  • Privacy Policy
  • Refund Policy
  • Mart Category
Partner
  • Partner Login
  • Partner Signup

Copyright © RVR Innovations LLP 2025 | All rights reserved - Mytat.co is the venture of RVR Innovations LLP