mysql primary key settings

MySQL is an open source relational database management system. In MySQL, a primary key is a column or combination of columns that uniquely identifies each row of data in a table. Primary keys play a vital role in tables, they improve retrieval speed and data integrity. The method of setting the primary key is very simple. This article will introduce how to set the primary key in MySQL.

1. What is a primary key?

The primary key is a column or a combination of columns used to uniquely identify each row of data in a table. The value of the primary key must be unique and cannot have duplicate values. Primary keys can also be used to speed up data retrieval and ensure data integrity. In MySQL, you can use a single column primary key or a composite primary key, depending on the design requirements of each table.

2. How to set the primary key

Setting the primary key in MySQL is very simple. When creating a table, just add the PRIMARY KEY keyword after the column definition.

For example, the following SQL statement creates a table named table1, in which the id column is the primary key:

CREATE TABLE table1 (
  id int NOT NULL PRIMARY KEY,
  name varchar(50) NOT NULL,
  age int NOT NULL
);

This SQL statement creates a table with three columns, in which the id column is the primary key, and the name and age columns are ordinary columns. As you can see, just add the PRIMARY KEY keyword after the id column definition, and the id column becomes the primary key.

In addition, when creating a table, you can also use multiple columns to define a composite primary key. For example, the following SQL statement creates a table named table2, in which the id column and the email column are combined into a composite primary key:

CREATE TABLE table2 (
  id int NOT NULL,
  email varchar(50) NOT NULL,
  name varchar(50) NOT NULL,
  PRIMARY KEY (id,email)
);

In this SQL statement, use "(id, email)" to represent id The column and email column together form a composite primary key.

3. How to add a primary key

If you forget to add a primary key when creating the table, you can also use the ALTER TABLE statement to add a primary key to the table.

The syntax is as follows:

ALTER TABLE table_name ADD PRIMARY KEY (column_name1, column_name2, ...);

For example, assuming we want to add a primary key to the table2 table mentioned above, we can use the following SQL statement:

ALTER TABLE table2 ADD PRIMARY KEY (id, email);

This SQL statement will id column and email column are set as primary keys.

It should be noted that when adding a primary key to a table, you must ensure that the data type of the column is not NULL.

4. How to modify the primary key

If you need to modify the primary key, you can use the ALTER TABLE statement to modify the primary key in the table.

The syntax is as follows:

ALTER TABLE table_name DROP PRIMARY KEY, ADD PRIMARY KEY (column_name1, column_name2, ...);

For example, assuming we want to change the primary key of the table1 table mentioned above from the id column to the name column, we can use the following SQL statement:

ALTER TABLE table1 DROP PRIMARY KEY, ADD PRIMARY KEY (name);

This SQL statement removes the id column from the primary key and then adds the name column to the primary key.

It should be noted that this operation may affect the data in the table, so you must back up the data before modifying the primary key.

5. Summary

It is very important to set the primary key in MySQL, which can improve the data retrieval speed and ensure data integrity. Setting the primary key is very simple. You only need to add the PRIMARY KEY keyword after the column definition, or use the ALTER TABLE statement to add or modify the primary key. Using primary keys allows us to better manage data.

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