MySQL Primary Key
Summary: in this tutorial, you will learn how to use MySQL primary key constraint to create the primary key for a table.
Introduction to MySQL primary key
A primary key is a column or a set of columns that uniquely identifies each row in the table. The primary key follows these rules:
- A primary key must contain unique values. If the primary key consists of multiple columns, the combination of values in these columns must be unique.
- A primary key column cannot have
NULLvalues. Any attempt to insert or updateNULLto primary key columns will result in an error. Note that MySQL implicitly adds aNOT NULLconstraint to primary key columns. - A table can have one an only one primary key.
Because MySQL works faster with integers, the data type of the primary key column should be the integer e.g., INT, BIGINT. And you should ensure sure that value ranges of the integer type for the primary key are sufficient for storing all possible rows that the table may have.
A primary key column often has the AUTO_INCREMENT attribute that automatically generates a sequential integer whenever you insert a new row into the table.
When you define a primary key for a table, MySQL automatically creates an index called PRIMARY.
MySQL PRIMARY KEY examples
The PRIMARY KEY constraint allows you to define a primary key of a table when you create or alter table.
1) Define a PRIMARY KEY constraint in CREATE TABLE
Typically, you define the primary key for a table in the CREATE TABLE statement.
If the primary key has one column, you can use the PRIMARY KEY constraint as a column constraint:
CREATE TABLE table_name(
primary_key_column datatype PRIMARY KEY,
...
);Code language: SQL (Structured Query Language) (sql)
When the primary key has more than one column, you must use the PRIMARY KEY constraint as a table constraint.
CREATE TABLE table_name(
primary_key_column1 datatype,
primary_key_column2 datatype,
...,
PRIMARY KEY(column_list)
);Code language: SQL (Structured Query Language) (sql)
In this syntax, you separate columns in the column_list by commas (,).
The PRIMARY KEY table constraint can be used when the primary key has one column:
CREATE TABLE table_name (
primary_key_column datatype,
... ,
PRIMARY KEY(primary_key_column)
);Code language: SQL (Structured Query Language) (sql)
The following example creates a table named users whose primary key is the user_id column:
CREATE TABLE users(
user_id INT AUTO_INCREMENT PRIMARY KEY,
username VARCHAR(40),
password VARCHAR(255),
email VARCHAR(255)
);Code language: SQL (Structured Query Language) (sql)
This statement creates the roles table that has the PRIMARY KEY constraint as the table constraint:
CREATE TABLE roles(
role_id INT AUTO_INCREMENT,
role_name VARCHAR(50),
PRIMARY KEY(role_id)
);Code language: SQL (Structured Query Language) (sql)
In case the primary key consists of multiple columns, you must specify them at the end of the CREATE TABLE statement. You put a comma-separated list of primary key columns inside parentheses followed the PRIMARY KEY keywords.
The following example creates the user_roles table whose primary key consists of two columns: user_id and role_id. It defines the PRIMARY KEY constraint as the table constraint:
CREATE TABLE user_roles(
user_id INT,
role_id INT,
PRIMARY KEY(user_id,role_id),
FOREIGN KEY(user_id)
REFERENCES users(user_id),
FOREIGN KEY(role_id)
REFERENCES roles(role_id)
);Code language: SQL (Structured Query Language) (sql)
Note that the statement also created two foreign key constraints.
2) Define PRIMARY KEY constraints using ALTER TABLE
If a table, for some reasons, does not have a primary key, you can use the ALTER TABLEstatement to add a primary key to the table as follows:
ALTER TABLE table_name
ADD PRIMARY KEY(column_list);Code language: SQL (Structured Query Language) (sql)
The following example adds the id column to the primary key.
First, create the pkdemos table without a primary key.
CREATE TABLE pkdemos(
id INT,
title VARCHAR(255) NOT NULL
);Code language: SQL (Structured Query Language) (sql)
Second, add a primary key to the pkdemos table using the ALTER TABLE statement:
ALTER TABLE pkdemos
ADD PRIMARY KEY(id);Code language: SQL (Structured Query Language) (sql)
If you add a primary key to a table that already has data. The data in the column(s), which will be included in the primary key, must be unique and not NULL.
PRIMARY KEY vs. UNIQUE KEY vs. KEY
KEY is the synonym for INDEX. You use the KEY when you want to create an index for a column or a set of columns that is not the part of a primary key or unique key.
A UNIQUE index ensures that values in a column must be unique. Unlike the PRIMARY index, MySQL allows NULL values in the UNIQUE index. In addition, a table can have multiple UNIQUE indexes.
Suppose that email and username of users in the users table must be unique. To enforce thes rules, you can define UNIQUE indexes for the email and username columns as the following statement:
Add a UNIQUE index for the username column:
ALTER TABLE users
ADD UNIQUE INDEX username_unique (username ASC) ;Code language: SQL (Structured Query Language) (sql)
Add a UNIQUE index for the email column:
ALTER TABLE users
ADD UNIQUE INDEX email_unique (email ASC) ;Code language: SQL (Structured Query Language) (sql)
In this tutorial, you have learned how to create a primary key for a new table or add a primary key to an existing table.