Adding Column with CASE in sub-select

Question

I have a sub-select within a larger query and I'm trying to add a calculated field to the sub-select using a CASE statement.

My sub-select looks like this:

LEFT JOIN 
(
    SELECT Course_ID, Question_ID, Answer, (HIP =
        CASE
            WHEN Question_ID = 77 THEN "1"
            WHEN Question_ID = 78 THEN "2"
            WHEN Question_ID = 79 THEN "3"
            WHEN Question_ID = 80 THEN "4"
            WHEN Question_ID = 81 THEN "5"
            WHEN Question_ID = 82 THEN "6"
            ELSE "Undefined HIP"
        END)
    FROM proposals.new_gen_ed_answers
    WHERE Answer = 1
) ercehip
    ON course.Course_ID = ercehip.Course_ID

When I try to access ercehip.HIP in my parent select statement though, it says it is undefined. Any ideas on how to solve this? Thanks

Answer 1

• Instead of using HIP = (..) , you will need to use Aliasing instead.
• Also, since all your CASE .. WHEN() are checking against Question_ID only, you can move it to besides Case clause, and make the query less verbose.

Try the following instead:

(
    SELECT Course_ID, Question_ID, Answer, 
        (CASE Question_ID
            WHEN 77 THEN "1"
            WHEN 78 THEN "2"
            WHEN 79 THEN "3"
            WHEN 80 THEN "4"
            WHEN 81 THEN "5"
            WHEN 82 THEN "6"
            ELSE "Undefined HIP"
        END) AS HIP 
    FROM proposals.new_gen_ed_answers
    WHERE Answer = 1
) AS ercehip

Answer 2

You must determinate alias of the calculated field as MySQL syntax require

SELECT Course_ID, Question_ID, Answer, (
    CASE
        WHEN Question_ID = 77 THEN "1"
        WHEN Question_ID = 78 THEN "2"
        WHEN Question_ID = 79 THEN "3"
        WHEN Question_ID = 80 THEN "4"
        WHEN Question_ID = 81 THEN "5"
        WHEN Question_ID = 82 THEN "6"
        ELSE "Undefined HIP"
    END) as HIP
FROM proposals.new_gen_ed_answers
WHERE Answer = 1

OR

SELECT Course_ID, Question_ID, Answer, (
    CASE
        WHEN Question_ID = 77 THEN "1"
        WHEN Question_ID = 78 THEN "2"
        WHEN Question_ID = 79 THEN "3"
        WHEN Question_ID = 80 THEN "4"
        WHEN Question_ID = 81 THEN "5"
        WHEN Question_ID = 82 THEN "6"
        ELSE "Undefined HIP"
    END) HIP
FROM proposals.new_gen_ed_answers
WHERE Answer = 1