A2-03-04.DDL-MySQL Primary Key

转载自:http://www.mysqltutorial.org/mysql-primary-key/

MySQL Primary Key

 

Summary: in this tutorial, you will learn how to use MySQL primary key constraint to create a primary key for the table.

Introduction to MySQL primary key

mysql primary keyA primary key is a column or a set of columns that uniquely identifies each row in the table. You must follow the rules below when you define a primary key for a table:

  • 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 contain NULL values. It means that you have to declare the primary key column with the NOT NULL  attribute. If you don’t, MySQL will force the primary key column as NOT NULL  implicitly.
  • A table has 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.You can choose a smaller integer type: TINYINTSMALLINT, etc. However, you should make sure that the range of values of the integer type for the primary key is sufficient for storing all possible rows that the table may have.

A primary key column often has the AUTO_INCREMENT attribute that generates a unique sequence for the key automatically. The primary key of the next row is greater than the previous one.

MySQL creates an index named PRIMARY with PRIMARY  type for the primary key in a table.

Defining MySQL PRIMARY KEY Constraints

MySQL allows you to create a primary key by defining a primary key constraint when you create or modify the table.

Defining MySQL PRIMARY KEY constraints using CREATE TABLE statement

MySQL allows you to create the primary key when you create the table using the CREATE TABLEstatement. To create a PRIMARY KEY  constraint for the table, you specify the PRIMARY KEY in the primary key column’s definition.

The following example creates users table whose primary key is user_id column:

You can also specify the PRIMARY KEY at the end of the CREATE TABLE  statement as follows:

In case the primary key consists of multiple columns, you must specify them at the end of the CREATE TABLE  statement. You put a coma-separated list of primary key columns inside parentheses followed the PRIMARY KEY  keywords.

Besides creating the primary key that consists of user_id and role_id columns, the statement also created two foreign key constraints.

Defining MySQL PRIMARY KEY constraints using ALTER TABLE statement

If a table, for some reasons, does not have a primary key, you can use the ALTER TABLE statement to add a column that has all necessary primary key’s characteristics to the primary key as the following statement:

The following example adds the id column to the primary key.

First, create the t1 table  without defining the primary key.

Second, make the id  column as the primary key of the t1 table.

PRIMARY KEY vs. UNIQUE KEY vs. KEY

KEY is a 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.

UNIQUE index creates a constraint for a column whose values must be unique. Unlike the PRIMARYindex, MySQL allows NULL values in the UNIQUE index. A table can also have multiple UNIQUE indexes.

For example, the email and username of a user in the users table must be unique. You can define UNIQUE indexes for the email and username columns as the following  statement:

Add a UNIQUE index for the username column.

Add a UNIQUE index for the email column.

In this tutorial, you have learned how to create a primary key for a new table or add a primary key for an existing table.

原文地址:https://www.cnblogs.com/zhuntidaoren/p/9523449.html