mysql怎么写关联语句

SELECT column_name(s)
FROM table1
JOIN table2 ON table1.column_name = table2.column_name;

-- 创建 employees 表
CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    department_id INT
);

-- 创建 departments 表
CREATE TABLE departments (
    id INT PRIMARY KEY,
    name VARCHAR(100)
);

-- 插入示例数据
INSERT INTO employees (id, name, department_id) VALUES (1, 'Alice', 1), (2, 'Bob', 2);
INSERT INTO departments (id, name) VALUES (1, 'HR'), (2, 'Engineering');

SELECT employees.name AS employee_name, departments.name AS department_name
FROM employees
JOIN departments ON employees.department_id = departments.id;

SELECT DISTINCT employees.name AS employee_name, departments.name AS department_name
FROM employees
JOIN departments ON employees.department_id = departments.id;

-- 在 department_id 上创建索引
CREATE INDEX idx_department_id ON employees(department_id);