Join 2 tables on dynamically changing column

Question

Having these 2 tables (from inventory functionalities)

SQL Fiddle

-- ins table
+------+-------------+-------------+
| id   |  direction  |   quantity  |
+------+-------------+-------------+
|  1   |     in      |      5      |
|  2   |     in      |      3      |
+------+-------------+-------------+


-- outs table
+------+-------------+-------------+
| id   |  direction  |   quantity  |
+------+-------------+-------------+
|  1   |     out     |      4      |
|  2   |     out     |      1      |
|  3   |     out     |      2      |
|  4   |     out     |      1      |
+------+-------------+-------------+

How can I join rows from outs table to a row from ins table that has quantity covers/equals to the quantities of the outs rows that joined it, in other words how to get a result like this ?

-- result
+------+-------------+-------------+------+-------------+-------------+
| id   |  direction  |   quantity  |  id  |  direction  | quantity    |
+------+-------------+-------------+------+-------------+-------------+
|  1   |     out     |      4      |  1   |     in      |      5      |
|  2   |     out     |      1      |  1   |     in      |      5      |
|  3   |     out     |      2      |  2   |     in      |      3      |
|  4   |     out     |      1      |  2   |     in      |      3      |
+------+-------------+-------------+------+-------------+-------------+

as you can see rows 1,2 from outs table is taken from/ joined to row 1 from ins table and rows 3,4 from outs table is taken from/ joined to row 2 from ins table

NOTE: the quantities in the 2 tables are guaranteed to be sealed (a row from ins table is always has quantity that is exactly equal to 1 or more quantities of rows from table outs)

I wish I can just do something like this

-- sedu SQL
SELECT 
    whatever 
FROM 
    outs left join 
    ins on outs.quantity <= (ins.quantity - previously joined outs.quantities);

Answer 1

This is painful to do in MySQL for a couple of reasons. First, MySQL doesn't have very good support for cumulative sums, which is what you want to compare.

And second, your result set is a little bit weak. It makes more sense to show all the ins records that contribute to each outs record, not just one of them.

For this purpose, you can use a join on cumulative sums, which looks like this:

select o.*, (o.to_quantity  - o.quantity) as from_quantity,
       i.*
from (select o.*,
             (select sum(o2.quantity)
              from outs o2
              where o2.id <= o.id
             ) as to_quantity
      from outs o
     ) o join
     (select i.*,
             (select sum(i2.quantity)
              from ins i2
              where i2.id <= i.id
             ) as to_quantity
      from ins i
     ) i
     on (o.to_quantity  - o.quantity) < i.to_quantity and
        o.to_quantity > (i.to_quantity  - i.quantity)

Here is the SQL Fiddle.

Answer 2

Subquery with correlation approach might also useful

select t.id, t.direction, t.quantity, i.id, i.direction, i.quantity  
from (
      select id, direction, quantity, 
             quantity + coalesce((select quantity from outs where id < o.id order by id desc limit 1),
                      (select quantity from outs where id > o.id order by id limit 1)) Qty
      from outs o
)t inner join ins i on i.quantity = t.Qty