CASE Statement Force an Error Message as a Condition - sql-server

Question

I want to implement this sort of logic:
select
many_columns,
case
when something then 'whatever'
else Generate Error Message and Stop the Query
end as [whatever]
from
table_1
Is this possible?
Edit #1:
Based on the comments that I got, I did some experimenting.
drop table if exists #testing;
go
with whatever as (
select 1 as [num] union
select 2 union
select 3 union
select 4 union
select 5 union
select 6 union
select 7 union
select 8 union
select 9 union
select 10
)
declare #counter INT
set #counter = 0
while (#counter <= (select count(*) from #testing))
begin
IF (#counter) < 5
print 'hi'
set #counter = #counter + 1
else
select * from #testing
print 'bye'
select #counter = #counter + 1
end
go
Hmm...the If statement is having issues with the else part...all I get is "Incorrect Syntax".

Answer 1

Seems the answer to my question is to combine a while loop with an if statement AFTER producing my main query to find the error message.
The final if statement will look something like this:
declare #counter INT
set #counter = 0
while (#counter <= (select count(*) from #testing))
begin
--print #counter
IF (#counter = (select [num] from #testing where [num] = #counter))
--print 'hi'
print cast(#counter as varchar) + ' ' + 'hi'
else
--select * from #testing where [num] = #counter
--print 'bye'
print cast(#counter as varchar) + ' ' + 'bye'
select #counter = #counter + 1
end
go

Answer 2

I can't put it better than the comment from #marc_s:
CASE in T-SQL is an expression (like a+b) that returns a single, atomic value. It is NOT a construct to deal with program flow and such - for that, you need to use IF / ELSE
👍
So let's take your original CASE expression code:
case
when something then 'whatever'
else Generate Error Message and Stop the Query
end as [whatever]
As an IF/ELSE (using BEGIN/END) flow:
IF call_that_a_knife = 'yes'
BEGIN
SELECT 'This is a knife' AS yes_i_call_that_a_knife;
END
ELSE
BEGIN
RAISERROR ('That's not a knife!', 16, 1);
END
;
RAISERROR()

Answer 3

The way I have implemented this kind of construct is by attempting to perform an addition operation with the error message in case the condition for the error is true:
select
many_columns,
1 + case
when <error_condition>
then 'Error Message Here will Stop the Query'
else 0 end as [success]
from
table_1
Assuming that the <error_condition> uses data from the rows fetched from table_1 or calls out some function that returns a value that may mean an error condition, this will run the query until it reaches the point the condition is true and then will error out with something like:
Msg 245, Level 16, State 1, Line 15
Conversion failed when converting the varchar value 'Error Message Here will Stop the Query' to data type int.