如果你C表有多行数据就不行吧。要么限定只返回一行
select a.col1,a.col2,b.col1 from a,b where a.col3=b.col3 and a.col1=(select top 1 col1 from c)
要么就改用in吧
select a.col1,a.col2,b.col1 from a,b where a.col3=b.col3 and a.col1 in (select col1 from c)