高手们请进，vb连接sql语句有关问题(2)

select * from a where day(ddate)=4
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select * from a where convert(char(10),ddate,21)= '2007-05-14 '

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不要将时间日期变成字符串来比较，因为字符串运算是最慢的。

Dim mdate As DateTime, sSQL As String
mdate = CDate( "2007-05-15 ")
sSQL = "SELECT * FROM a WHERE ddate > = mdate and ddate < mdate + 1 "
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为什么不用datadiff？