MySQL与MAX的分组不能按预期工作？

Question

I have a table:

ID | User | Amount
1  | 1    | 50
2  | 1    | 80
3  | 2    | 80
4  | 2    | 100
5  | 1    | 90
6  | 1    | 120
7  | 2    | 120
8  | 1    | 150
9  | 2    | 300

I do a query:

SELECT * FROM TABLE ORDER BY amount DESC group by userid

I'm getting this:

ID | User | Amount
1  | 1    | 50
2  | 1    | 80

But I was expecting:

ID | User | Amount
9  | 2    | 300
8  | 1    | 150

What is wrong with my sql?

Answer 1

When grouping you have to use aggregate functions like max() for all columns that are not grouped by

select t.* 
from table t
inner join 
(
   SELECT userid, max(amount) as total
   FROM TABLE 
   group by userid
) x on x.userid = t.userid and x.total = t.amount
ORDER BY t.amount DESC 

Answer 2

Another solution.Check SQL Fiddle Using FIND_IN_SET clause

SELECT
  ua.*
FROM user_amount ua
WHERE FIND_IN_SET(ua.amount,(SELECT
                               MAX(ua1.amount)
                             FROM user_amount ua1
                             WHERE ua1.user = ua.user)) > 0
ORDER BY amount desc;  

Using IN clause

SELECT
  ua.*
FROM user_amount ua
WHERE ua.amount IN (SELECT
                               MAX(ua1.amount)
                             FROM user_amount ua1
                             WHERE ua1.user = ua.user)
ORDER BY amount desc