select * from A where date between +@ymd + ' 00:00:00' and @ymd +' 23:59:59'

------解决方案--------------------
SQL code

select * from A where Dates between @ymd+' 00:00:00' and @ymd +' 23:59:59'
or
create proc p_test
@ymd varchar(12)
as
declare @sql varchar(max),@sql2 varchar(max)
set @sql='
select * from A where date between '''+@ymd + ' 00:00:00'''+'and ''' +@ymd +' 23:59:59'''
print @sql---select * from A where date between '2012-04-18 00:00:00'and '2012-04-18 23:59:59'