???楼主说的是类似 from tbname t 这样子吧!没什么特别的,只是为了方便,t是表的别名,查询的或条件里用到该表的字段,可以用t取代表名,方便些。
Select * from SC as SC_2 where SC_2.S#=SC.S# and SC_2.C#='002'select avg(SC_2.score) from SC SC_2 where SC_2.C#=SC.C#就像这种,这完全可以不用别名啊第二个,直接 select avg(SC_2.score) from SC 就可以了吗,为什么要这么用了
第一个和Select * from SC where SC.C#='002' 有什么分别啊,一个表名,一个别名,同样的表,联合查询,为什么要这么操作啊
不一定要按你找到的去书写,那只是得到需要结果集的一种方式而已。例如 select * from DM_DeliveryOrder a join DM_DeliveryOrderDetail b on a.id = b.deliveryOrderId这种,如果不加别名,那么条件里的表名就会有一大串的字符,看起来不是很直观。
update SC set score=(select avg(SC_2.score) from SC SC_2 where SC_2.C#=SC.C# ) from Course,Teacher where Course.C#=SC.C# and Course.T#=Teacher.T# 同样的问题,还有,update 后面可以接from子句吗,from Course,Teacher
update后是可以接from的,类似: update b set b.createtime = getdate() from DM_DeliveryOrder a join DM_DeliveryOrderDetail b on a.id = b.deliveryOrderId where a.listid = 'LS00000001'更新DM_DeliveryOrderDetail表的createtime字段。
只是将某个表做两个表来连接查询,类似:有个表 employee 字段有 id name parentid(父级id)要查询员工张三的信息及上级负责人的信息则:select * from employee a join employee b on a.id = b.parentid where b.[name] = '张三'
这和 select * from employee id = parentid where name = '张三' and id = parentid 有什么区别了,这种做法,能否根据 Select * from SC as SC_2 where SC_2.S#=SC.S# and SC_2.C#='002' 这个典型的例句讲清楚一点了, 因为如果是我,我一定这样查 Select * from SC where C#='002'大脑是不是少根弦啊
Select * from SC as SC_2 where SC_2.S#=SC.S# and SC_2.C#='002'select avg(SC_2.score) from SC SC_2 where SC_2.C#=SC.C#就像这种,这完全可以不用别名啊第二个,直接 select avg(SC_2.score) from SC 就可以了吗,为什么要这么用了
select *
from DM_DeliveryOrder a join DM_DeliveryOrderDetail b on a.id = b.deliveryOrderId这种,如果不加别名,那么条件里的表名就会有一大串的字符,看起来不是很直观。
from SC SC_2
where SC_2.C#=SC.C# ) from Course,Teacher where Course.C#=SC.C# and Course.T#=Teacher.T#
同样的问题,还有,update 后面可以接from子句吗,from Course,Teacher
update b
set b.createtime = getdate()
from DM_DeliveryOrder a join DM_DeliveryOrderDetail b on a.id = b.deliveryOrderId
where a.listid = 'LS00000001'更新DM_DeliveryOrderDetail表的createtime字段。
字段有 id name parentid(父级id)要查询员工张三的信息及上级负责人的信息则:select *
from employee a join employee b on a.id = b.parentid
where b.[name] = '张三'
select *
from employee id = parentid
where name = '张三' and id = parentid
有什么区别了,这种做法,能否根据
Select * from SC as SC_2 where SC_2.S#=SC.S# and SC_2.C#='002'
这个典型的例句讲清楚一点了,
因为如果是我,我一定这样查
Select * from SC where C#='002'大脑是不是少根弦啊