MySQL实现7种 SQL JOINS
UNION 和 UNION ALL 关键字使用
MySQL在实现满外连接的时候，并不能像Oracle一样直接使用FULL OUTER JOIN,因此需要使用 UNION 和 UNION ALL 两个关键字，UNION 和 UNION ALL 使用语法如下
SELECT column, .... from table1
UNION [ALL]
SELECT column, .... from table2
UNION操作符
UNION 操作符返回两个查询的结果集的并集，去除重复记录。
UNION ALL操作符
UNION ALL操作符返回两个查询的结果集的并集。对于两个结果集的重复部分，不去重。
注意：执行UNION ALL语句时所需要的资源比UNION语句少。如果明确知道合并数据后的结果数据 不存在重复数据，或者不需要去除重复的数据，则尽量使用UNION ALL语句，以提高数据查询的效率。
7种SQL JOIN 实现
数据库在进行多表查询的时候，应该会有以下七种JOIN情况如下图所示
MySQL具体实现案例如下
创建一张员工表和一张部门表，表结构如图
员工表
部门表
实现连接的SQL
## 中图 内连接
SELECT employee_id,department_name
FROM employees e JOIN departments d
ON e.department_id = d.department_id
## 左上图 左外连接
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.department_id = d.department_id
# 右上图 右外连接
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.department_id = d.department_id
## 左中图
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.department_id = d.department_id
WHERE d.department_id IS NULL
## 右中图
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.department_id = d.department_id
WHERE e.department_id IS NULL
## 左下图: 满外连接
# 方式1: 左上图 UNION ALL 右中图
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.department_id = d.department_id
UNION ALL
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.department_id = d.department_id
WHERE e.department_id IS NULL
# 方式2: 左中图 UNION ALL 右上图
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.department_id = d.department_id
WHERE d.department_id IS NULL
UNION ALL
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.department_id = d.department_id
# 方式3 左上图 UNION 右上图 （不推荐使用 UNION，效率低）
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.department_id = d.department_id
UNION
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.department_id = d.department_id
## 右下图 左中图 UNION ALL 右中图
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.department_id = d.department_id
WHERE d.department_id IS NULL
UNION ALL
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.department_id = d.department_id
WHERE e.department_id IS NULL