php - How do I efficiently compare users? -

i have users can "like" categories. instance, may have 2 users:

john likes apples, oranges, pears

bob likes apples, oranges, pie, cake

they both apples, oranges

this isn't issue 2 users, when imagine scaling thousands of users, thousands of likes, there major efficiency concerns.

i need able compare user other users, , determine likes have in common.

i have tried array_intersect, not scale. need mysql solution.

how efficiently return users share same likes, , likes shared?

users +-------+-------------+------+-----+---------+----------------+ | field | type        | null | key | default |          | +-------+-------------+------+-----+---------+----------------+ | id    | int(11)     | no   | pri | null    | auto_increment | | name  | varchar(16) | no   |     | null    |                | +-------+-------------+------+-----+---------+----------------+  categories +-------+-------------+------+-----+---------+----------------+ | field | type        | null | key | default |          | +-------+-------------+------+-----+---------+----------------+ | id    | int(11)     | no   | pri | null    | auto_increment | | name  | varchar(32) | no   |     | null    |                | +-------+-------------+------+-----+---------+----------------+  likes +-------------+---------+------+-----+---------+-------+ | field       | type    | null | key | default | | +-------------+---------+------+-----+---------+-------+ | user_id     | int(11) | no   | mul | null    |       | | category_id | int(11) | no   | mul | null    |       | +-------------+---------+------+-----+---------+-------+ 

function find_intersect($likes1, $likes2){      sort($likes1);     sort($likes2);      $intersect = array();      $i = 0;     $j = 0;      while ($i < count($likes1) , $j < count($likes2)){         if ($likes1[$i] == $likes2[$j]){             array_push($intersect, $likes1[$i]);             $i++;             $j++;             }         else if ($likes1[$i] < $likes2[$j])             $i++;         else             $j++;     }      return $intersect; } 

above drummed , should efficient way of finding intersection of 2 arrays. agree @danfarrell though in mysql or database way more efficient in managing information when comes thousands of users.