Question

Table Users : user_id , credits_available
Table Survey : user_id, survey_id, credits_required

Now I want to calculate the difference between credits_available and credits_required for a particular user_id and specified survey_id ? Is it possible to write a single query instead of calculating separately by 2 different queries?

Answer 1

MySQL lets you use normal math…

SELECT (survey.credits_required - users.credits_available) as endResult
FROM users,survey 
WHERE users.user_id = survey.user_id 
AND survey_id = $surveyID
AND users.user_id = $userID

And of course, $surveyID is a variable which lists the survey ID in question… $userID is a variable which lists the user ID in question…