LEFT JOIN以后记录数变少了

[jronald]

SELECT a.gid,avg10,avg20 FROM 
(SELECT gid,avg10=(first_volt+last_volt)*0.5 FROM ak000515_10) a LEFT JOIN 
(SELECT gid,avg20=(first_volt+last_volt)*0.5 FROM ak000515_20) b ON (b.gid+1)*2-1=a.gid 
WHERE a.gid BETWEEN 324 AND 382 

没问题，显示了324到382条记录

SELECT a.gid,avg10,avg20 FROM 
(SELECT gid,avg10=(first_volt+last_volt)*0.5 FROM ak000515_10) a LEFT JOIN 
(SELECT gid,avg20=(first_volt+last_volt)*0.5 FROM ak000515_20) b ON (b.gid+1)*2-1=a.gid LEFT JOIN 
(SELECT gid,avg50=(first_volt+last_volt)*0.5 FROM ak000515_50) c ON (c.gid+1)*5-1=a.gid 
WHERE a.gid BETWEEN 324 AND 382 

乱了，少了很多，而且不连续了，这是怎么回事？WHERE子句去掉就没问题，想快一些，所以先定a.gid的范围，再做LEFT JOIN，要怎么写啊？

[zheninchangjiang]

the different between 'where' and 'and'

[水族杰纶]

条件不一样,导致的~~

[jronald]

[Quote=引用 2 楼 wufeng4552 的回复:]
条件不一样,导致的~~
[/Quote]

就是多了一个LEFT JOIN，WHERE一样啊，那记录应该是从324到382最，而且连续(表中这些记录都有存在的)。

[jronald]

哎，原来是我没排序，加个ORDER BY a.gid就可以了，送分了。