Question

Good day,

I have a MySQL table which has some duplicate rows that have to be removed while adding a value from one column in the duplicated rows to the original.

The problem was caused when another column had the wrong values and that is now fixed but it left the balances split among different rows which have to be added together. The newer rows that were added must then be removed.

In this example, the userid column determines if they are duplicates (or triplicates). userid 6 is duplicated and userid 3 is triplicated.

As an example for userid 3 it has to add up all balances from rows 3, 11 and 13 and has to put that total into row 3 and then remove rows 11 and 13. The balance columns of both of those have to be added together into the original, lower ID row and the newer, higher ID rows must be removed.

ID | balance | userid
---------------------
1  | 10      | 1
2  | 15      | 2
3  | 300     | 3
4  | 80      | 4
5  | 0       | 5
6  | 65      | 6
7  | 178     | 7
8  | 201     | 8
9  | 92      | 9
10 | 0       | 10
11 | 140     | 3
12 | 46      | 6
13 | 30      | 3

I hope that is clear enough and that I have provided enough info. Thanks =)

Answer 1

Two steps.

1. Update:

    UPDATE 
        tableX AS t
      JOIN
        ( SELECT userid
               , MIN(id) AS min_id
               , SUM(balance) AS sum_balance
          FROM tableX
          GROUP BY userid
        ) AS c
        ON  t.userid = c.userid
    SET
        t.balance = CASE WHEN t.id = c.min_id 
                        THEN c.sum_balance
                        ELSE 0 
                    END ;

2. Remove the extra rows:

    DELETE t
    FROM
        tableX AS t
      JOIN
        ( SELECT userid
               , MIN(id) AS min_id
          FROM tableX
          GROUP BY userid
        ) AS c
        ON  t.userid = c.userid
        AND t.id > c.min_id
    WHERE
        t.balance = 0 ;

---

Once you have this solved, it would be good to add a UNIQUE constraint on userid as it seems you want to be storing the balance for each user here. That will avoid any duplicates in the future. You could also remove the (useless?) id column.