Question

How can I select just some rows in the following table so that they sum up to certain value?

Table
-----
id | qty1 | qty2 | qty3 | qty4
------------------------------
1  | 0.0  | 0.0  | 10   | 20
2  | 1.5  | 0.0  | 7.5  | 18
3  | 1.0  | 2.0  | 7.5  | 18
4  | 0.0  | 0.5  | 5    | 13

Let’s say, the top value I want is 57…

So I need to select the rows from the previous table such that qty1+qty2+qty3+qty4 of each row, get until that 57 value, and discard the other rows. In this example, I would get the following:

id | qty1 | qty2 | qty3 | qty4
------------------------------
1  | 0.0  | 0.0  | 10   | 20
2  | 1.5  | 0.0  | 7.5  | 18

Because 10+20+1.5+7.5+18 = 57, so I discard rows 3 & 4…

Now I wish that the top value is 50, then I should get:

id | qty1 | qty2 | qty3 | qty4
------------------------------
1  | 0.0  | 0.0  | 10   | 20
2  | 1.5  | 0.0  | 7.5  | 11

Since these values sum up to 50, and the 7 from row2,qty4 is left out…
(BTW the rows are ordered in this particular way because that’s the order in which I wish to account for the sums of qtys… It’s not valid to sum up first row1, then 3, then 2 then 4, for example… They should always be sumed in the order 1,2,3,4…)

What if I would like the complement of this? I mean, the other two rows I didn’t got in the last result.

First case:

id | qty1 | qty2 | qty3 | qty4
------------------------------
3  | 1.0  | 2.0  | 7.5  | 18
4  | 0.0  | 0.5  | 5    | 13

Second case:

id | qty1 | qty2 | qty3 | qty4
------------------------------
2  | 0.0  | 0.0  | 0.0  | 7
3  | 1.0  | 2.0  | 7.5  | 18
4  | 0.0  | 0.5  | 5    | 13

(If the second case is too complicated, how about obtaining:

id | qty1 | qty2 | qty3 | qty4
------------------------------
1  | 0.0  | 0.0  | 10   | 20

Because adding up the original qtys of row 2 would surpass the 50 value, I discard it…
The complement in this case should just be:

id | qty1 | qty2 | qty3 | qty4
------------------------------
2  | 1.5  | 0.0  | 7.5  | 18
3  | 1.0  | 2.0  | 7.5  | 18
4  | 0.0  | 0.5  | 5    | 13

)

Answer 1

Let’s put it this way: If SQL was a religion I’d go to hell for providing this solution. SQL is not meant to solve this kind of problems, so any solution would be horrible. Mine is no exception 🙂

set @limitValue := 50;
select id, newQty1, newQty2, newQty3, newQty4 from (
  select id,
  if(@limitValue - qty1 > 0, qty1, greatest(@limitValue, 0)) newQty1,
  @limitValue := @limitValue - qty1 Total1,
  if(@limitValue - qty2 > 0, qty2, greatest(@limitValue, 0)) newQty2,
  @limitValue := @limitValue - qty2 Total2,
  if(@limitValue - qty3 > 0, qty3, greatest(@limitValue, 0)) newQty3,
  @limitValue := @limitValue - qty3 Total3,
  if(@limitValue - qty4 > 0, qty4, greatest(@limitValue, 0)) newQty4,
  @limitValue := @limitValue - qty4 Total4
  from (
    select id, qty1, qty2, qty3, qty4,
      @rowTotal < @limitValue Useful,
      @previousRowTotal := @rowTotal PreviousRowTotal,
      @rowTotal := @rowTotal + qty1 + qty2 + qty3 + qty4 AllRowsTotal,
      @rowTotal - @previousRowTotal CurrentRowTotal 
    from t,
    (select @rowTotal := 0, @previousRowTotal := 0) S1
  ) MarkedUseful
  where useful = 1
) Final

For the provided data, this results in:

+----+---------+---------+---------+---------+
| ID | NEWQTY1 | NEWQTY2 | NEWQTY3 | NEWQTY4 |
+----+---------+---------+---------+---------+
|  1 | 0       |       0 | 10      |      20 |
|  2 | 1.5     |       0 | 7.5     |      11 |
+----+---------+---------+---------+---------+

And the complement:

set @limitValue := 50;
select t1.id,
  coalesce(t1.qty1 - newQty1, t1.qty1) newQty1,
  coalesce(t1.qty2 - newQty2, t1.qty2) newQty2,
  coalesce(t1.qty3 - newQty3, t1.qty3) newQty3,
  coalesce(t1.qty4 - newQty4, t1.qty4) newQty4
from t t1 left join (
    select id,
    if(@limitValue - qty1 > 0, qty1, greatest(@limitValue, 0)) newQty1,
    @limitValue := @limitValue - qty1 Total1,
    if(@limitValue - qty2 > 0, qty2, greatest(@limitValue, 0)) newQty2,
    @limitValue := @limitValue - qty2 Total2,
    if(@limitValue - qty3 > 0, qty3, greatest(@limitValue, 0)) newQty3,
    @limitValue := @limitValue - qty3 Total3,
    if(@limitValue - qty4 > 0, qty4, greatest(@limitValue, 0)) newQty4,
    @limitValue := @limitValue - qty4 Total4
    from (
      select id, qty1, qty2, qty3, qty4,
        @rowTotal < @limitValue Useful,
        @previousRowTotal := @rowTotal PreviousRowTotal,
        @rowTotal := @rowTotal + qty1 + qty2 + qty3 + qty4 AllRowsTotal,
        @rowTotal - @previousRowTotal CurrentRowTotal 
      from t,
      (select @rowTotal := 0, @previousRowTotal := 0) S1
    ) MarkedUseful
    where useful = 1
) Final
on t1.id = final.id
where Total1 < 0 or Total2 < 0 or Total3 < 0 or Total4 < 0 or final.id is null

For the provided data, this results in:

+----+---------+---------+---------+---------+
| ID | NEWQTY1 | NEWQTY2 | NEWQTY3 | NEWQTY4 |
+----+---------+---------+---------+---------+
|  2 |       0 | 0       | 0       |       7 |
|  3 |       1 | 2       | 7.5     |      18 |
|  4 |       0 | 0.5     | 5       |      13 |
+----+---------+---------+---------+---------+

Enjoy!