SQL query to match a comma-separated string against a comma-separated string?

Question

The MySQL query below uses PHP to pull in the $sector, which is a single digit, and the $subsector_text, which is a comma separated string. The $subsector_text could be a

Answer 1

It's not practical to match any value in a comma-separate string against any value in another comma-separated string in a single predicate.

You can use FIND_IN_SET() to search for one value at a time.

This means you need multiple predicates, one for each value you get by splitting your input $subsector_text. So split your variable and map it into a series of FIND_IN_SET() calls.

I haven't tested the following code, but it should give you the idea of what I'm talking about:

$subsector_array = array_map('intval', explode(',', $subsector_text));
$subsector_terms = array_map(
  function ($id) { return "FIND_IN_SET($id, a.subsector)"; },
  $subsector_array);
$subsector_expr = implode(' OR ', $subsector_terms);

$sql = "
SELECT ...
          WHERE a.state = 1 
            AND a.sector = '$sector'
            AND ($subsector_expr)
...";

This will of course force a table-scan because there's no way to index FIND_IN_SET(), or any other operation that searches for substrings. Well, I suppose your conditions on a.state and a.sector will use an index to narrow down the search before applying the FIND_IN_SET() conditions.

I understand the dilemma of having to work with a system that you inherited. Let your manager know that this needs to get refactored at some point, because it will never be efficient or reliable the way it's designed now.