(3)使用group by 时有关约束条件的问题
如果约束条件是单个字段,则必须用where,并且where要放在group by之前,如:
select cityname from MI where vdatetime>=? and vdatetime<? group by cityname如果约束条件是统计函数,则要使用having,并且having要放在group by之后,如:(2)
(4)其实,在某些场合,group by可以用distinct代替,如以下两条语句功能相同:
select cityname from MI where vdatetime>=? and vdatetime<? group by citynameselect distinct cityname from MI where vdatetime>=? and vdatetime<?
2.在count()中使用别名问题
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