mysql - 加入不要覆盖结果 mysql

Question

如果连接表中的行不存在，是否有避免覆盖的方法？如果最后连接的表行为 NULL，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

你想要一个 FULL OUTER JOIN...mysql 不支持，所以你必须 UNION 一个 LEFT OUTER JOIN 右外连接

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/