532 lines
11 KiB
Markdown
532 lines
11 KiB
Markdown
# MySQL UNION 语法使用文档
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## 目录
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1. [UNION 基础语法](#union-基础语法)
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2. [UNION 规则和限制](#union-规则和限制)
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3. [示例数据准备](#示例数据准备)
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4. [UNION 基础使用示例](#union-基础使用示例)
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5. [UNION vs UNION ALL](#union-vs-union-all)
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6. [复杂查询示例](#复杂查询示例)
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7. [最佳实践和注意事项](#最佳实践和注意事项)
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## UNION 基础语法
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UNION 用于合并两个或多个 SELECT 语句的结果集。
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```sql
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SELECT column1, column2, ... FROM table1
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UNION [ALL]
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SELECT column1, column2, ... FROM table2
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[UNION [ALL]
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SELECT column1, column2, ... FROM table3]
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...
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[ORDER BY column_name]
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```
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## UNION 规则和限制
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1. **列数相同**：每个 SELECT 语句必须拥有相同数量的列
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2. **数据类型兼容**：对应位置的列必须具有兼容的数据类型
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3. **列名**：结果集使用第一个 SELECT 语句的列名
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4. **去重**：UNION 默认去除重复记录，UNION ALL 保留所有记录
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5. **ORDER BY**：只能在最后使用，对整个结果集排序
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## 示例数据准备
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### 创建示例表
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```sql
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-- 创建员工表
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CREATE TABLE employees (
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id INT PRIMARY KEY,
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name VARCHAR(50),
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department VARCHAR(30),
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salary DECIMAL(10,2),
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city VARCHAR(30)
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);
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-- 创建前员工表
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CREATE TABLE former_employees (
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id INT PRIMARY KEY,
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name VARCHAR(50),
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department VARCHAR(30),
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last_salary DECIMAL(10,2),
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city VARCHAR(30),
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leave_date DATE
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);
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-- 创建管理层表
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CREATE TABLE managers (
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id INT PRIMARY KEY,
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name VARCHAR(50),
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level VARCHAR(20),
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salary DECIMAL(10,2),
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region VARCHAR(30)
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);
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```
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### 插入示例数据
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```sql
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-- 插入员工数据
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INSERT INTO employees (id, name, department, salary, city) VALUES
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(1, '张三', '技术部', 8000.00, '北京'),
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(2, '李四', '销售部', 6000.00, '上海'),
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(3, '王五', '技术部', 9000.00, '深圳'),
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(4, '赵六', '人事部', 5500.00, '北京'),
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(5, '钱七', '财务部', 7000.00, '广州'),
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(6, '孙八', '技术部', 8500.00, '杭州'),
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(7, '周九', '市场部', 6500.00, '成都');
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-- 插入前员工数据
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INSERT INTO former_employees (id, name, department, last_salary, city, leave_date) VALUES
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(8, '吴十', '技术部', 7500.00, '北京', '2023-08-15'),
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(9, '郑一', '销售部', 5800.00, '上海', '2023-09-20'),
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(10, '王五', '技术部', 8000.00, '深圳', '2023-07-10'),
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(11, '刘二', '人事部', 5200.00, '广州', '2023-10-05');
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-- 插入管理层数据
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INSERT INTO managers (id, name, level, salary, region) VALUES
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(12, '陈总', '高级经理', 15000.00, '华北'),
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(13, '林总', '部门经理', 12000.00, '华东'),
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(14, '张三', '项目经理', 10000.00, '华南'),
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(15, '黄总', '区域经理', 11000.00, '华西');
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```
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## UNION 基础使用示例
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### 1. 简单合并查询
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合并当前员工和前员工的姓名：
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```sql
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SELECT name FROM employees
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UNION
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SELECT name FROM former_employees;
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```
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**结果：**
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```
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+------+
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| name |
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+------+
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| 张三 |
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| 李四 |
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| 王五 |
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| 赵六 |
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| 钱七 |
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| 孙八 |
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| 周九 |
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| 吴十 |
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| 郑一 |
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| 刘二 |
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+------+
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```
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### 2. 多列合并查询
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合并员工和管理层的姓名和薪资：
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```sql
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SELECT name, salary FROM employees
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UNION
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SELECT name, salary FROM managers
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ORDER BY salary DESC;
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```
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**结果：**
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```
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+------+----------+
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| name | salary |
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+------+----------+
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| 陈总 | 15000.00 |
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| 林总 | 12000.00 |
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| 黄总 | 11000.00 |
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| 张三 | 10000.00 |
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| 王五 | 9000.00 |
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| 孙八 | 8500.00 |
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| 张三 | 8000.00 |
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| 钱七 | 7000.00 |
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| 周九 | 6500.00 |
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| 李四 | 6000.00 |
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| 赵六 | 5500.00 |
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+------+----------+
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```
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### 3. 使用别名统一列名
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```sql
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SELECT name, department AS work_area, salary FROM employees
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UNION
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SELECT name, region AS work_area, salary FROM managers
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ORDER BY work_area;
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```
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**结果：**
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```
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+------+-----------+----------+
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| name | work_area | salary |
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+------+-----------+----------+
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| 钱七 | 财务部 | 7000.00 |
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| 陈总 | 华北 | 15000.00 |
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| 林总 | 华东 | 12000.00 |
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| 张三 | 华南 | 10000.00 |
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| 黄总 | 华西 | 11000.00 |
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| 赵六 | 人事部 | 5500.00 |
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| 李四 | 销售部 | 6000.00 |
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| 周九 | 市场部 | 6500.00 |
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| 张三 | 技术部 | 8000.00 |
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| 王五 | 技术部 | 9000.00 |
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| 孙八 | 技术部 | 8500.00 |
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+------+-----------+----------+
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```
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## UNION vs UNION ALL
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### UNION（去重）
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```sql
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SELECT city FROM employees
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UNION
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SELECT city FROM former_employees;
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```
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**结果：**
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```
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+------+
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| city |
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+------+
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| 北京 |
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| 上海 |
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| 深圳 |
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| 广州 |
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| 杭州 |
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| 成都 |
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+------+
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```
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### UNION ALL（保留重复）
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```sql
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SELECT city FROM employees
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UNION ALL
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SELECT city FROM former_employees;
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```
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**结果：**
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```
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+------+
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| city |
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+------+
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| 北京 |
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| 上海 |
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| 深圳 |
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| 北京 |
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| 广州 |
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| 杭州 |
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| 成都 |
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| 北京 |
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| 上海 |
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| 深圳 |
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| 广州 |
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+------+
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```
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### 性能对比示例
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```sql
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-- 统计重复记录数量
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SELECT
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'UNION' AS query_type,
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COUNT(*) AS record_count
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FROM (
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SELECT city FROM employees
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UNION
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SELECT city FROM former_employees
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) AS union_result
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UNION ALL
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SELECT
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'UNION ALL' AS query_type,
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COUNT(*) AS record_count
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FROM (
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SELECT city FROM employees
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UNION ALL
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SELECT city FROM former_employees
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) AS union_all_result;
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```
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**结果：**
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```
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+------------+--------------+
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| query_type | record_count |
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+------------+--------------+
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| UNION | 6 |
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| UNION ALL | 11 |
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+------------+--------------+
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```
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## 复杂查询示例
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### 1. 条件过滤与UNION
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查询高薪员工和所有管理层：
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```sql
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SELECT name, salary, '高薪员工' AS category
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FROM employees
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WHERE salary > 8000
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UNION
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SELECT name, salary, '管理层' AS category
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FROM managers
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ORDER BY salary DESC;
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```
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**结果：**
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```
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+------+----------+----------+
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| name | salary | category |
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+------+----------+----------+
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| 陈总 | 15000.00 | 管理层 |
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| 林总 | 12000.00 | 管理层 |
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| 黄总 | 11000.00 | 管理层 |
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| 张三 | 10000.00 | 管理层 |
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| 王五 | 9000.00 | 高薪员工 |
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| 孙八 | 8500.00 | 高薪员工 |
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+------+----------+----------+
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```
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### 2. 聚合查询与UNION
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各部门薪资统计：
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```sql
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SELECT department AS name, AVG(salary) AS avg_salary, '部门平均' AS type
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FROM employees
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GROUP BY department
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UNION ALL
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SELECT '公司总体' AS name, AVG(salary) AS avg_salary, '总体平均' AS type
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FROM employees
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UNION ALL
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SELECT '管理层' AS name, AVG(salary) AS avg_salary, '管理平均' AS type
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FROM managers
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ORDER BY avg_salary DESC;
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```
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**结果：**
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```
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+----------+-------------+----------+
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| name | avg_salary | type |
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+----------+-------------+----------+
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| 管理层 | 12000.00000 | 管理平均 |
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| 技术部 | 8500.00000 | 部门平均 |
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| 公司总体 | 7142.85714 | 总体平均 |
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| 财务部 | 7000.00000 | 部门平均 |
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| 市场部 | 6500.00000 | 部门平均 |
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| 销售部 | 6000.00000 | 部门平均 |
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| 人事部 | 5500.00000 | 部门平均 |
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+----------+-------------+----------+
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```
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### 3. 多表复杂联合查询
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创建完整的人员名册：
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```sql
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SELECT
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id,
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name,
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department,
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salary,
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city,
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'在职' AS status
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FROM employees
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UNION ALL
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SELECT
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id,
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name,
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department,
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last_salary AS salary,
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city,
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'离职' AS status
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FROM former_employees
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UNION ALL
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SELECT
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id,
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name,
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'管理层' AS department,
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salary,
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region AS city,
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'管理' AS status
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FROM managers
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ORDER BY status, salary DESC;
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```
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### 4. 使用子查询的UNION
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查询每个城市的最高薪资员工：
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```sql
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SELECT name, city, salary, '当前最高薪' AS note
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FROM employees e1
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WHERE salary = (
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SELECT MAX(salary)
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FROM employees e2
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WHERE e2.city = e1.city
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)
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UNION
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SELECT name, city, last_salary AS salary, '前员工最高薪' AS note
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FROM former_employees f1
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WHERE last_salary = (
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SELECT MAX(last_salary)
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FROM former_employees f2
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WHERE f2.city = f1.city
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)
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ORDER BY salary DESC;
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```
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## 最佳实践和注意事项
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### 1. 性能优化建议
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```sql
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-- ❌ 避免：不必要的UNION，可以用OR替代
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SELECT * FROM employees WHERE department = '技术部'
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UNION
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SELECT * FROM employees WHERE department = '销售部';
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-- ✅ 推荐：使用OR条件
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SELECT * FROM employees
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WHERE department IN ('技术部', '销售部');
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```
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### 2. 数据类型兼容性
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```sql
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-- ✅ 正确：确保数据类型兼容
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SELECT id, name, salary FROM employees
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UNION
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SELECT id, name, CAST(last_salary AS DECIMAL(10,2)) FROM former_employees;
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-- ❌ 错误：数据类型不兼容可能导致错误
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SELECT id, name, salary FROM employees
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UNION
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SELECT id, name, leave_date FROM former_employees; -- leave_date是DATE类型
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```
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### 3. 使用索引优化
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```sql
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-- 为UNION查询中的过滤条件创建索引
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CREATE INDEX idx_employees_dept ON employees(department);
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CREATE INDEX idx_employees_salary ON employees(salary);
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CREATE INDEX idx_former_employees_dept ON former_employees(department);
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-- 优化后的查询
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SELECT name, department FROM employees WHERE department = '技术部'
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UNION
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SELECT name, department FROM former_employees WHERE department = '技术部';
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```
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### 4. 内存使用注意事项
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```sql
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-- 对于大型结果集，考虑使用LIMIT
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SELECT name, salary FROM employees
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UNION ALL
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SELECT name, salary FROM managers
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ORDER BY salary DESC
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LIMIT 10;
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```
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### 5. 常见错误和解决方案
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```sql
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-- ❌ 错误：列数不匹配
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SELECT name, department FROM employees
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UNION
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SELECT name FROM managers; -- 缺少一列
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-- ✅ 修正：补齐列数
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SELECT name, department FROM employees
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UNION
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SELECT name, level AS department FROM managers;
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-- ❌ 错误：ORDER BY位置错误
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SELECT name FROM employees ORDER BY name
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UNION
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SELECT name FROM managers ORDER BY name;
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-- ✅ 修正：ORDER BY只能在最后
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SELECT name FROM employees
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UNION
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SELECT name FROM managers
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ORDER BY name;
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```
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### 6. 实用查询模式
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#### 数据完整性检查
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```sql
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-- 检查是否有重复的员工记录
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SELECT name, COUNT(*) as count
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FROM (
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SELECT name FROM employees
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UNION ALL
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SELECT name FROM former_employees
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UNION ALL
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SELECT name FROM managers
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) AS all_people
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GROUP BY name
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HAVING COUNT(*) > 1;
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```
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#### 数据对比分析
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```sql
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-- 对比不同表中的数据分布
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SELECT
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'当前员工' AS source,
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department,
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COUNT(*) AS count,
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AVG(salary) AS avg_salary
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FROM employees
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GROUP BY department
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UNION ALL
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SELECT
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'前员工' AS source,
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department,
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COUNT(*) AS count,
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AVG(last_salary) AS avg_salary
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FROM former_employees
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GROUP BY department
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ORDER BY source, department;
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```
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---
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**总结：**
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- UNION 是合并查询结果的强大工具
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- 注意列数、数据类型的匹配
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- 根据需求选择 UNION 或 UNION ALL
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- 合理使用索引和LIMIT提升性能
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- 避免不必要的复杂UNION操作 |