SQL query corresponding to the maximum value of a category table field
A, SQL queries, a category table corresponding to the field values of the maximum ( operation on a table ):
Question: According to the teacher id query checkOutAutoID a record of the largest in the balance?
explore:
1.group by method:
First lookup table corresponding to the same teacherID checkOutAutoID maxima:
select MAX(checkOutAutoID) from T_CheckOut group by teacherID order by teacherID
The maximum value is then isolated (assuming that the isolated 3), the corresponding isolated remainCash:
SELECT remainCash FROM T_CheckOut WHERE checkOutAutoID=3
Merge SQL:
SELECT remainCash FROM T_CheckOut WHERE checkOutAutoID=(select MAX(checkOutAutoID) from T_CheckOut group by teacherID order by teacherID)
The results given:
Solution:
Join top 1 in the sub-query can be:
SELECT top 1 remainCash FROM T_CheckOut WHERE checkOutAutoID=(select top 1 MAX(checkOutAutoID) from T_CheckOut group by teacherID order by teacherID)
Two data library, run time 0.035s.
2. Nesting:
select remainCash,teacherID,checkOutAutoID from T_CheckOut as a
where checkOutAutoID=(select max(b.checkOutAutoID)
from T_CheckOut as b
where a.teacherID = b.teacherID
)
Curry two data, the running time 0.146s, the above method is relatively efficient is low, over 5 times.
That question is, our checkOutAutoID is unique, if we change the idea, assume checkOutAutoID is not unique, we might find out the checkOutAutoID same record had a lot of pieces, but as long as one of us how to do it? Just do a little modification OK:
select * from
(select remainCash,teacherID,checkOutAutoID from T_CheckOut as a
where checkOutAutoID=(select max(b.checkOutAutoID)
from T_CheckOut as b
where a.teacherID = b.teacherID
)
)as a
group by checkOutAutoID
二、根据卡表中的卡号，插入学生表一条记录（卡表与学生表存在外键关联）：
insert into T_Student(cardAutoID,stuID,stuName,sex)select cardAutoID,@stuID,@stuName,@sexfrom T_Card where cardID=@cardID
总结：
group by方法相对嵌套来说，查询效率高，语句简单，不易出错，所以还是建议使用第一种方法。
感谢您的阅读！
