group by 到底有什么作用?
hanml 2006-01-20 11:45:05 有一条sql语句,大家帮忙解释一下啊
select orderinfo0_.order_no as col_0_0_,
orderinfo0_.order_prefix as col_1_0_,
customer1_.cust_name as col_2_0_,
customer1_.cust_code as col_2_1_
from ORDER orderinfo0_
inner join CUSTOMER customer1_
on orderinfo0_.cust_code=customer1_.cust_code
where orderinfo0_.order_no='OR-00000019'
group by orderinfo0_.order_no
为什么这样查出的结果有很多条order_no都是OR-00000019的记录?
其实的ORDER 表中只有一条 order_no是OR-00000019的记录,和CUSTOMER表join后应该只有一条记录啊,可是加了group by 后就有很多条记录,如果去掉group by就只有一条,这是为什么?