从多个值中选择最高值

Question

I have this table

trans

id
user_id
amt
desc
bal
created

sample data is

id  user_id desc    amt     bal     created
1   1   credit  12.00   12.00   2013-07-02 00:00:00
2   1   credit  44.00   56.00   2013-07-09 00:00:00
3   1   debit   11.00   45.00   2013-07-18 00:00:00
4   2   credit  11.00   11.00   2013-07-12 01:17:08
5   2   credit  45.00   56.00   2013-07-13 01:17:08
6   2   debit   15.00   41.00   2013-07-14 01:17:08
7   3   credit  33.00   33.00   2013-07-15 01:17:52
8   3   credit  45.00   78.00   2013-07-16 01:17:52
9   3   debit   25.00   53.00   2013-07-17 01:17:52

now i need to select the current balance from each user only, so outout should be output id user_id desc amt bal created 3 1 debit 11.00 45.00 2013-07-18 00:00:00 6 2 debit 15.00 41.00 2013-07-12 01:17:08 9 3 credit 25.00 53.00 2013-07-12 01:17:52

this is what i tried

SELECT * 
FROM trans
WHERE created
IN (

SELECT MAX( created ) 
FROM trans
GROUP BY user_id
)

this gives two records for each user instead of 1 each. what am i doing wrong?

The goal is the select Top amounts (highest and descending)

it is ok if two dates and time are same, but not returning two records each

Answer 1

I find this simpler if you use a JOIN :

SELECT t.*
FROM trans t
   INNER JOIN (
       SELECT Max(Id) MaxId
       FROM trans
       GROUP BY User_Id
   ) t2 ON t.Id = t2.MaxId

This assumes the MAX(Id) is the max record you're looking for with each user (and is unique). If not, you can use MAX(created) with the User_Id the same way, but it could yield multiple results per user if the time is the same. For example:

SELECT t.*
FROM trans t
   INNER JOIN (
       SELECT Max(created) MaxCreated, User_Id
       FROM trans
       GROUP BY User_Id
   ) t2 ON t.User_Id = t2.User_Id AND t.Created = t2.MaxCreated

Answer 2

A good solution is;

SELECT user_id, SUM(IF(desc='credit',-1,1)*amt) AS balance 
FROM trans GROUP BY user_id

Your suggested table is not well designed. You should really not store the balance for each row, as that can introduce inconsistency in your data. What happens if the individual amt don't agree with the bal fields? Store just the amt fields. It would be simpler if you dropped the desc column and stored credits as negative. In that case, you just select SUM(amt) AS balance .