Menu
×
   ❮     
HTML CSS JAVASCRIPT SQL PYTHON JAVA PHP HOW TO W3.CSS C C++ C# BOOTSTRAP REACT MYSQL JQUERY EXCEL XML DJANGO NUMPY PANDAS NODEJS R TYPESCRIPT ANGULAR GIT POSTGRESQL MONGODB ASP AI GO KOTLIN SASS VUE DSA GEN AI SCIPY AWS CYBERSECURITY DATA SCIENCE
     ❯   

MySQL Tutorial

MySQL HOME MySQL Intro MySQL RDBMS

MySQL SQL

MySQL SQL MySQL SELECT MySQL WHERE MySQL AND, OR, NOT MySQL ORDER BY MySQL INSERT INTO MySQL NULL Values MySQL UPDATE MySQL DELETE MySQL LIMIT MySQL MIN and MAX MySQL COUNT, AVG, SUM MySQL LIKE MySQL Wildcards MySQL IN MySQL BETWEEN MySQL Aliases MySQL Joins MySQL INNER JOIN MySQL LEFT JOIN MySQL RIGHT JOIN MySQL CROSS JOIN MySQL Self Join MySQL UNION MySQL GROUP BY MySQL HAVING MySQL EXISTS MySQL ANY, ALL MySQL INSERT SELECT MySQL CASE MySQL Null Functions MySQL Comments MySQL Operators

MySQL Database

MySQL Create DB MySQL Drop DB MySQL Create Table MySQL Drop Table MySQL Alter Table MySQL Constraints MySQL Not Null MySQL Unique MySQL Primary Key MySQL Foreign Key MySQL Check MySQL Default MySQL Create Index MySQL Auto Increment MySQL Dates MySQL Views

MySQL References

MySQL Data Types MySQL Functions

MySQL Examples

MySQL Examples MySQL Editor MySQL Quiz MySQL Exercises MySQL Certificate

MySQL PRIMARY KEY Constraint


MySQL PRIMARY KEY Constraint

The PRIMARY KEY constraint uniquely identifies each record in a table.

Primary keys must contain UNIQUE values, and cannot contain NULL values.

A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).


PRIMARY KEY on CREATE TABLE

The following SQL creates a PRIMARY KEY on the "ID" column when the "Persons" table is created:

CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    PRIMARY KEY (ID)
);

To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:

CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    CONSTRAINT PK_Person PRIMARY KEY (ID,LastName)
);

Note: In the example above there is only ONE PRIMARY KEY (PK_Person). However, the VALUE of the primary key is made up of TWO COLUMNS (ID + LastName).



PRIMARY KEY on ALTER TABLE

To create a PRIMARY KEY constraint on the "ID" column when the table is already created, use the following SQL:

ALTER TABLE Persons
ADD PRIMARY KEY (ID);

To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:

ALTER TABLE Persons
ADD CONSTRAINT PK_Person PRIMARY KEY (ID,LastName);

Note: If you use ALTER TABLE to add a primary key, the primary key column(s) must have been declared to not contain NULL values (when the table was first created).


DROP a PRIMARY KEY Constraint

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

ALTER TABLE Persons
DROP PRIMARY KEY;

×

Contact Sales

If you want to use W3Schools services as an educational institution, team or enterprise, send us an e-mail:
sales@w3schools.com

Report Error

If you want to report an error, or if you want to make a suggestion, send us an e-mail:
help@w3schools.com

W3Schools is optimized for learning and training. Examples might be simplified to improve reading and learning. Tutorials, references, and examples are constantly reviewed to avoid errors, but we cannot warrant full correctness of all content. While using W3Schools, you agree to have read and accepted our terms of use, cookie and privacy policy.

Copyright 1999-2024 by Refsnes Data. All Rights Reserved. W3Schools is Powered by W3.CSS.