MySQL-DDL-table structure operation

DDL (table operations)

• table creation

  • Show how to create a data table with the display of specific code

  • CREATE DATABASE <database name>;
    
    CREATE TABLE <table name> (
      <column name 1> <data type 1> <constraint>,
      <column name 2> <data type 2> <constraint>,
      ...
    )

  • Specific code example:

    • create table tb_user
      (
          id       int comment '用户ID,唯一标识',
          username varchar(20) comment '用户名',
          name     char(20) comment '姓名',
          age      int comment '年龄',
          gender   char(1) comment '性别'
      ) comment '用户信息表'

  • The result of the operation is as follows 
    • 
  •  Constraints: Constraints are rules that act on fields in a table to limit the data stored in the table

    • 1. PRIMARY KEY: Used to specify the column as the primary key to ensure its uniqueness and non-nullness.

      2. UNIQUE: The value used to specify the column must be unique, but can be empty.

      3. NOT NULL: The value used to specify the column cannot be empty.

      4. DEFAULT: It is used to specify the default value of the column. When inserting a new row, if the value of the column is not provided, the default value will be used.

      5. FOREIGN KEY: Used to create a foreign key relationship, the specified column is associated with columns in other tables.

      6. CHECK: Used to specify the value range or condition of the column to ensure that the value of the column meets the specified condition.

  • Purpose: To ensure the correctness, validity and integrity of the database

  • Constraints created by the above code

    • The use of specific constraints is demonstrated in the form of code
      • Basic format: attribute name data type constraint
        • The specific code is as follows:

          • create table tb_user
            (
                id       int primary key comment '用户ID,唯一标识',
                username varchar(20) not null unique comment '用户名',
                name     char(20)    not null comment '姓名',
                age      int comment '年龄',
                gender   char(1) default '男' comment '性别'
            ) comment '用户信息表'

          • The result of the operation is as follows: 
          • 

The primary key in the data table generally adds an auto-increment attribute 

-- 查询所有数据库
create table tb_user
(
    id       int primary key auto_increment comment '用户ID,唯一标识',
    username varchar(20) not null unique comment '用户名',
    name     char(20)    not null comment '姓名',
    age      int comment '年龄',
    gender   char(1) default '男' comment '性别'
) comment '用户信息表'

•  type of data

  • There are many data types in MySQL, mainly three types: numeric type, string type, date type
    • value type
      • 
    • string type

       

      • 
    • date type
      •