SQL JOINS sql语句的关联查询

ps:mysql不支持FULL OUTER JOIN

so~

最后两种分别写成：

SELECT * FROM A LEFT JOIN B ON A.key = B.key
UNION
SELECT * FROM A RIGHT JOIN B ON A.key = B.key
           

SELECT * FROM A LEFT JOIN B ON A.key = B.key
WHERE B.key IS NULL
UNION
SELECT * FROM A RIGHT JOIN B ON A.key = B.key
WHERE A.key IS NULL