SQL中的7种 JOIN 操作
在 SQL 中，JOIN 操作用于将两个或多个表的数据结合起来。以下是 7 种常见的 JOIN 类型及其对应的 SQL 语法和示意图：

1. 左连接 (LEFT JOIN)

   
   SELECT <select_list>
   FROM TableA A
   LEFT JOIN TableB B
   ON A.Key = B.Key;

2. 右连接 (RIGHT JOIN)

   
   SELECT 
   FROM TableA A
   RIGHT JOIN TableB B
   ON A.Key = B.Key;

3. 内连接 (INNER JOIN)

   
   SELECT <select_list>
   FROM TableA A
   INNER JOIN TableB B
   ON A.Key = B.Key;

4. 左连接排除 (LEFT JOIN with IS NULL)

   
   SELECT 
   FROM TableA A
   LEFT JOIN TableB B
   ON A.Key = B.Key
   WHERE B.Key IS NULL;

5. 右连接排除 (RIGHT JOIN with IS NULL)

   
   SELECT <select_list>
   FROM TableA A
   RIGHT JOIN TableB B
   ON A.Key = B.Key
   WHERE A.Key IS NULL;

6. 完全外连接 (FULL OUTER JOIN)

   
   SELECT 
   FROM TableA A
   FULL OUTER JOIN TableB B
   ON A.Key = B.Key;

7. 完全外连接排除 (FULL OUTER JOIN with IS NULL)

   
   SELECT <select_list>
   FROM TableA A
   FULL OUTER JOIN TableB B
   ON A.Key = B.Key
   WHERE A.Key IS NULL OR B.Key IS NULL;

需要注意的是，FULL OUTER JOIN 在 MySQL 中不被直接支持，需要使用 UNION 来实现：

SELECT 
FROM TableA A
LEFT JOIN TableB B
ON A.Key = B.Key
UNION
SELECT 
FROM TableB B
LEFT JOIN TableA A
ON A.Key = B.Key;