如何获得总结为特定值的行的随机组合？

Question

The Table I have:

+--------+--------------------+
|item_id|        value        |
+--------+--------------------+
|  1     | 1                  |
|  2     | 4                  |
|  3     | 2                  |
|  4     | 6                  |
+--------+--------------------+

What the SQL Query Result should be: A random combination of items that sum up to 10 with a variable number of different items (in this case 2.)

+--------------+--------------------+-------------+
|item_id       |        amount      |      sum    |
+--------------+--------------------+-------------+
|2             |2                   |8            |
|3             |1                   |2            |
+--------------+--------------------+-------------+

The results show

You get 2 times item 2 (which has the value of 4 so it adds to a sum of 8).

And you get one time item 3 (which has the value of 2)

This combination sums up to 10.

Is that even possible, when it should not be the same combination always and picked random if there are other possibilitys?

Answer 1

Assuming you want a single random combination, you can do:

select
  *
from (
  select
    a.item_id as item1, 
    x.n as amount1, 
    a.value * x.n as sum1,
    b.item_id as item2, 
    y.n as amount2, 
    b.value * y.n as sum2,
    rand() as r
  from my_table a
  join my_table b on b.item_id <> a.item_id
  cross join (
    select 1 as n union select 2 union select 3 union select 4 
    union select 5 union select 6 union select 7 union select 8 
    union select 9 union select 10) x
  cross join (
    select 1 as n union select 2 union select 3 union select 4
    union select 5 union select 6 union select 7 union select 8 
    union select 9 union select 10) y
  where a.value * x.n + b.value * y.n = 10
) z
order by r -- sorted randomly
limit 1 -- to get only one combination; remove to get them all

Every time you run this query it picks a random [different] solution.

The script to create the table and data you mentioned (that I used to test) is:

create table my_table (
  item_id int,
  value int
);

insert into my_table (item_id, value) values (1, 1);
insert into my_table (item_id, value) values (2, 4);
insert into my_table (item_id, value) values (3, 2);
insert into my_table (item_id, value) values (4, 6);

EDIT on July 1st, 2019: As requested, here's an equivalent [shorter] solution using a recursive CTE (Common Table Expression), available in MariaDB since 10.2.2 (see Recursive Common Table Expressions):

with recursive
val as (select 1 as n union all select n + 1 from val where n < 10)
select
  *
from (
  select
    a.item_id as item1, 
    x.n as amount1, 
    a.value * x.n as sum1,
    b.item_id as item2, 
    y.n as amount2, 
    b.value * y.n as sum2,
    rand() as r
  from my_table a
  join my_table b on b.item_id <> a.item_id
  cross join val x
  cross join val y
  where a.value * x.n + b.value * y.n = 10
) z
order by r -- sorted randomly
limit 1 -- to get only one combination; remove to get all 22 answers

This solution scales much better if you need to use higher numbers.