mysql分組排序以及排序失效問題

一、要求

我們需要按公司分組，傳回最新的一條消息，理論上

SELECT
	A.* 
FROM
	( SELECT * FROM `bac_distribution` ORDER BY distribution_id DESC ) A 
GROUP BY
	A.customer
           

就可以了吧，實際上查到的卻是：

原因是mysql做了優化，直接将内查詢去掉了。

EXPLAIN EXTENDED 
SELECT
	A.* 
FROM
	( SELECT DISTINCT * FROM `bac_distribution` ORDER BY distribution_id DESC ) A 
GROUP BY
	A.customer;
SHOW WARNINGS;
           

兩行選中，一起檢視

二、解決方案

在裡面的查詢上做一點操作就可以了 DISTINCT LINMIT都可以，傾向于第一個

SELECT A.* FROM
(SELECT DISTINCT* FROM `bac_distribution`  ORDER BY distribution_id desc ) A
GROUP BY A.customer
           

SELECT A.* FROM
(SELECT * FROM `bac_distribution`  ORDER BY distribution_id desc limit 10000) A
GROUP BY A.customer