mysql建表时如何加约束

在 MySQL 中，可以使用以下语法来为表添加约束：

1. PRIMARY KEY 约束：用于定义主键。语法如下：

   CREATE TABLE table_name (
      column1 datatype constraints,
      column2 datatype constraints,
      ...
      PRIMARY KEY (one or more columns)
   );

示例：

   CREATE TABLE users (
      user_id INT AUTO_INCREMENT,
      username VARCHAR(50) NOT NULL,
      email VARCHAR(100) NOT NULL,
      password VARCHAR(50) NOT NULL,
      PRIMARY KEY (user_id)
   );

2. UNIQUE 约束：用于定义唯一键。语法如下：

   CREATE TABLE table_name (
      column1 datatype constraints,
      column2 datatype constraints,
      ...
      UNIQUE (one or more columns)
   );

示例：

   CREATE TABLE users (
      user_id INT AUTO_INCREMENT,
      username VARCHAR(50) NOT NULL UNIQUE,
      email VARCHAR(100) NOT NULL UNIQUE,
      password VARCHAR(50) NOT NULL,
      PRIMARY KEY (user_id)
   );

3. FOREIGN KEY 约束：用于定义外键。语法如下：

   CREATE TABLE table_name1 (
      column1 datatype constraints,
      column2 datatype constraints,
      ...
      PRIMARY KEY (one or more columns)
   );

   CREATE TABLE table_name2 (
      column1 datatype constraints,
      column2 datatype constraints,
      ...
      FOREIGN KEY (column_name) REFERENCES table_name1(column_name)
   );

示例：

   CREATE TABLE departments (
      department_id INT AUTO_INCREMENT,
      department_name VARCHAR(50) NOT NULL,
      location VARCHAR(100) NOT NULL,
      PRIMARY KEY (department_id)
   );

   CREATE TABLE employees (
      employee_id INT AUTO_INCREMENT,
      first_name VARCHAR(50) NOT NULL,
      last_name VARCHAR(50) NOT NULL,
      email VARCHAR(100) NOT NULL UNIQUE,
      department_id INT,
      FOREIGN KEY (department_id) REFERENCES departments(department_id),
      PRIMARY KEY (employee_id)
   );

以上是 MySQL 中常用的约束类型，可以根据需求选择适合的约束类型。