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标题: LEFT JION的用法 [打印本页]

作者: yuanzai32 时间: 2011-11-22 15:11
标题: LEFT JION的用法
SELECT IFNULL(SUM(skt43.skf610),0)-IFNULL(SUM(skt43.skf669),0),
skt57.skf772
FROM skt57
LEFT JOIN (skt43,skt42)
ON (skt43.skf616=skt42.skf587
AND skt57.skf772=skt42.skf589
AND skt42.skf591=5
AND skt43.skf611>=20110701
AND skt43.skf611<=20110930)
AND skt42.skf597=0
GROUP BY skt57.skf772,skt42.skf590
HAVING skt42.skf590=16;

SELECT IFNULL(SUM(skt43.skf610),0)-IFNULL(SUM(skt43.skf669),0),
skt57.skf772,
FROM skt43,skt42,skt57
WHERE skt43.skf616=skt42.skf587
AND skt57.skf772=skt42.skf589
AND skt42.skf590=16
AND skt42.skf591=5 AND SKT42.SKF597=0
AND skt43.skf611>=20110701
AND skt43.skf611<=20110930
GROUP BY skt57.skf772;

这两个实现的查询结果都是一样的，所以就有疑问了，这两种哪个好，百度了只有讲LEFT JOIN ON和LEFT JOIN WHERE的区别的，越学问题越多呀

作者: kider 时间: 2011-11-23 09:33
用explain看看就知道了
原则上先滤出数据排序比排序后再滤数据效果要好

作者: 永生 时间: 2011-11-23 10:54
用explain看看就知道了

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