join不要覆盖结果mysql

Question

如果连接表中的行不存在，是否有一种避免覆盖的方法？如果最后一个连接的表行为NULL，则它可以覆盖具有相同列名的其他表字段。

SELECT * FROM books
LEFT JOIN item ON books.item_id = item.item_id
LEFT JOIN stuff ON item.item_id = stuff.item_id //Not there, haha i will override item_id!
...

Answer 1

你想要FULL OUTER JOIN ... mysql不支持，所以你必须UNION LEFT OUTER JOIN RIGHT OUTER JOIN

SELECT * FROM books
LEFT OUTER JOIN item ON books.item_id = item.item_id
LEFT OUTER JOIN stuff ON item.item_id = stuff.item_id //Not there, haha i will override item_id!

UNION

SELECT * FROM books
RIGHT OUTER JOIN item ON books.item_id = item.item_id
RIGHT OUTER JOIN stuff ON item.item_id = stuff.item_id //Not there, haha i will override item_id!
...

http://www.sitepoint.com/understanding-sql-joins-mysql-database/