MySQL Select 3 random rows where sum of three rows is less than value

Question

I am trying to select three random rows from a table, where their combined item_price column is less than a desired amount.

Imagine you have an

Answer 1

Here's a SQL only (MySQL flavour) solution:

SELECT i.*
FROM items i
CROSS JOIN
    (SELECT CONCAT('^(', t1.item_id, '|', t2.item_id, '|', t3.item_id, ')$') AS regex
     FROM items t1
     CROSS JOIN items t2
     CROSS JOIN items t3
     WHERE t1.item_id < t2.item_id
       AND t2.item_id < t3.item_id 
       AND t1.item_price + t2.item_price + t3.item_price <= 300
     ORDER BY RAND()
     LIMIT 1) s
WHERE i.item_id REGEXP s.regex

Not very efficient for large result sets as it creates a subquery of the different permutations of 3 items that fulfill the total criteria and then picks one of these at random. The subquery returns its result as a regular expression to allow the rows to be picked in the outer query.

See SQL Fiddle demo.