SQL Query efficiency within class
Doing this in a loop will be slow, and doubling the calls by retrieving each column with a different query will make things worse.

Why not use a JOIN between your original query, something like this:-

SELECT database.job_id, job_sheet.qty, job_sheet.row
FROM database
INNER JOIN job_sheet
ON database.job_id = job_sheet.job_id

In your loop this would mean a single query rather than ~20000 queries.

If you do need to do a separate query for each time through the loop then a prepared statement would help but peformance will likely still be poor.