SQL Month comparison to declared variable - sql

Question

select
sc.locationid, --here to get a result
(
if month(date()) between 8 and 12 then #semTerm = 'S1'
else #semTerm = 'S2'
end if
)as #semTerm
from student_class sc
where #semTerm = sc.semester
;
In db2 student management system. Read only access. Desired outcome is If Jan thru June, S2 else if Aug thru Dec, S1. Trying to setup a variable based on the current date stamp where the month is segregated then assigned to a variable then compared against a column in student_class table.
Have tried case statements as well with no luck. Unable to declare #semTerm without error above select statement. Looked at where clause solution also. Am I out in left field? Seems simple but struggling with the syntax. Part of a larger statement with locationID as one column in student_class.

Answer 1

You can't really use an IF statement in a simple SELECT statement, you must use CASE:
select
sc.locationid, --here to get a result
case
when month(current date) between 8 and 12 then 'S1'
when month(current date) between 1 and 6 then 'S2'
else ''
end as semTerm
from
student_class sc
If you want to find only the students for the current semester, then you would want to move the CASE statement into the WHERE clause:
select
sc.locationid, --here to get a result
sc.semester,
...
from
student_class sc
where
sc.semester = case
when month(current date) between 8 and 12 then 'S1'
when month(current date) between 1 and 6 then 'S2'
end

Answer 2

A CASE expression is generally the way to implement conditional logic in a SELECT statement. However, it would be more efficient if you didn't have to recalculate it on every row, right?
One approach would be to "precalculate" it in a common table expression, and join to that as your selection criteria:
with v (semester) as
( values
case
when month(current date) > 7 then 'S1'
when month(current date) < 7 then 'S2'
else null
end
)
select
sc.locationid,
sc.semester,
...
from
student_class sc
join
v on sc.semester = v.semester;
OR
If you you find that this current semester value would be useful in many other places, another approach could be to create a 'global session variable' to hold the value. (except on z/OS)
create or replace variable
v_sememster char(2)
default
case
when month(current date) > 7
then 'S1'
when month(current date) < 7
then 'S2'
else null
end;
Then you can have a very simple query:
select
sc.locationid,
sc.semester,
...
from
student_class sc
where
sc.semester = v_semester;