Trouble with case statement - sql

Question

I am having problems with this case statement. I don't know what I am doing wrong but I get the error:
Only one expression can be specified in the select list when the subquery is not introduced with EXISTS.
I have a case when the field equals a value then do a left outer join but if the field equals a different value then do a inner join.
This is my query:
SELECT
case
when oqt = '26' then
(Select qm.id_oqm, cast(isNull(id_eval, '') as varChar(50)) + ' - ' + qm.methodName as methodName, qm.methodName as actualMethod,cv.*
FROM OQMethods QM left outer join courseversions cv on cv.evalid = QM.id_eval and cv.courselanguage = 'EN' and cv.courseactive='Y' and cv.id_cp > 0
WHERE QM.id_oqt in (SELECT tempoq.oqt FROM tempoq INNER JOIN OQMethods ON tempoq.oqt = OQMethods.id_oqt)and active = 1)
END,
case
when oqt = '31' then
(Select qm.id_oqm, cast(isNull(id_eval, '') as varChar(50)) + ' - ' + qm.methodName as methodName, qm.methodName as actualMethod,cv.*
FROM OQMethods QM inner join courseversions cv on cv.evalid = QM.id_eval and cv.courselanguage = 'EN' and cv.courseactive='Y' and cv.id_cp > 0
where QM.id_oqt in (SELECT tempoq.oqt FROM tempoq INNER JOIN OQMethods ON tempoq.oqt = OQMethods.id_oqt) and active = 1)
END
from tempoq

Answer 1

The case is an expression that must evaluate to a value. The Select statements that you have return multiple values.
It would seem that you're trying to use Case like it's a C# switch? If that's the case, then you're likely better off with an IF ELSE IF construction.

Answer 2

It looks like you want to do something like this rather than using a CASE statement.
DECLARE #t int
-- This would contain your oqt value
SET #t =1
IF #t = 1
BEGIN
SELECT * FROM tableA
END
ELSE IF #t = 2
BEGIN
SELECT * FROM TableB
END

Answer 3

Select qm.id_oqm, cast(isNull(id_eval, '') as varChar(50)) + ' - ' + qm.methodName as methodName, qm.methodName as actualMethod,cv.*
FROM OQMethods QM
inner join tempoq on tempoq.oqt = QM.id_oqt
left outer join courseversions cv on cv.evalid = QM.id_eval and cv.courselanguage = 'EN' and cv.courseactive='Y' and cv.id_cp > 0
WHERE active = 1 and (tempoq.oqt = '26' or (tempoq.oqt = '31' and courseversions.* is not null))
left outer join means join OQMethods's data which even no match data from courseversions,
then filter the data with null courseversions.* that is inner join.
Hope I have the right understanding.