How to set the primary key of the table in mysql

MySQL is a popular relational database management system that can be used to manage large amounts of data. In MySQL, in order to ensure the uniqueness and storage efficiency of data in the table, we need to set a primary key for the table. This article will introduce how to set the primary key of a table in MySQL.

1. What is a primary key

The primary key is a special field or combination of fields used to uniquely identify each record in the table. The function of the primary key is to prevent data duplication and improve query efficiency.

The primary key can be a combination of one or more fields. For example, if a person may have the same name, then you can use "name ID number" as the primary key, so that a person can be uniquely identified.

2. Set the primary key of the table

1. Set the primary key when creating the table

We need to set the primary key when creating the table. In MySQL, you can use the following syntax to create a table with a primary key:

CREATE TABLE 表名 (
     列名1 数据类型 NOT NULL,
     列名2 数据类型 NOT NULL,
     ...
     PRIMARY KEY (列名1,列名2,...)
);

In the above syntax, column name 1, column name 2, etc. represent the column names in the table, and the data type represents the data type of the column. NOT NULL defines that the column cannot be empty, and PRIMARY KEY (column name 1, column name 2,...) specifies that the column is the primary key.

2. Add a primary key

If a table has been created but no primary key has been set, we can use the following syntax to add a primary key to the table:

ALTER TABLE 表名 
ADD PRIMARY KEY (列名1,列名2,...);

In the above syntax, ALTER TABLE is used to modify the table structure, ADD PRIMARY KEY is used to add a primary key, and column name 1, column name 2, etc. represent the column names in the table.

3. Precautions for the primary key

1. Uniqueness of the primary key

The primary key must be unique. There can only be one primary key in a table, and The value of the primary key cannot be NULL.

2. The role of the primary key

The primary key can be used to perform associated query and update operations on data. Querying using the primary key will be faster. Therefore, when designing, the role and purpose of the primary key need to be taken into consideration.

3. Selection of primary key

The primary key should select a unique and non-duplicate field or field combination. When selecting a primary key, you also need to consider whether the column will be updated or queried frequently, because the frequency of updates and queries will affect the performance of the database.

4. The size of the primary key

We should try to make the length of the primary key as small as possible, because a long primary key will reduce the query efficiency and storage efficiency of the data.

In short, setting the primary key is a very important concept in a relational database. Accurate primary key design can eliminate duplicate data, improve the efficiency of data query and update, and make the database performance better. When designing the table structure, we should choose the primary key reasonably so that it can play the greatest role in the application.

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