select bargain_name,bargain_owner_IdCard 
from t_bargain 
where bargain_owner_IdCard=(SELECT SUBSTRING(bargain_owner_IdCard, 0, 18)
            FROM  dbo.t_bargain where bargain_name = '200900010')

上面的查询没有结果，但SELECT SUBSTRING(bargain_owner_IdCard, 0, 18)FROM  dbo.t_bargain where bargain_name = '200900010' 是有结果的：362502197505180816。请教大家substring函数在sql语句中的用法规则是怎样的。