RAISERROR within Case statement - tsql

Question

Can you not raise errors within a case statement in T-SQL? I always have problems with SQL case statements :/
begin try
declare #i int
--set #i = (select COUNT(1) from table_name)
select Item_Num =
CASE (select COUNT(1) from table_name)
when 1 then (select Item_Num from table_name)
when 0 then (raiserror('No records in database', 0, 0))
ELSE (raiserror('Multiple records in database', 0, 0))
END
from table_name
end try
begin catch
declare #errormsg nvarchar(1024),
#severity int,
#errorstate int;
select #errormsg = error_message(),
#severity = error_severity(),
#errorstate = error_state();
raiserror(#errormsg, #severity, #errorstate);
end catch

Answer 1

Think of Case/When as operating on a single piece of data. If you think of it this way, a lot of your problems will go away.
If/Then is used to control the flow of logic.
Something like this should work for you.
declare #i int
set #i = (select COUNT(1) from table_name)
If #i = 1
Begin
Print "1 row"
End
Else If #i = 0
Begin
Print "no rows"
End
Else
Begin
Print "too many rows"
End

Answer 2

You can raise an error from the case expression by converting an error string to int.
select case (select count(*) from mytable)
when 1 then 100
when 2 then 200
else convert(int, 'ERROR')
end
This gives an error message like
Conversion failed when converting the varchar value 'ERROR' to data type int.
which is about as good as you're going to get.
Not all failed conversions give the input string in the error message. Conversions to datetime, for example, do not. So if your case expression returns a datetime, you still have to trigger the error with a string-to-integer conversion:
select case (select count(*) from mytable)
when 1 then getdate()
else convert(datetime, convert(int, 'ERROR'))
end
It gets worse: if you are returning a date, you can't explicitly convert that from int, so you have to resort to
convert(date, convert(char(1), convert(int, 'ERROR')))
It's pretty horrible, but in my opinion the only thing more important than clean code is informative error messages, so I live with it.

Answer 3

As I said in the comment, I think it would be easier to simply create a flag that you check outside the scope of the CASE statement. Something along the lines of:
--- code before the TRY...
BEGIN TRY
DECLARE #i int
-- declare a variable to act as a flag
DECLARE #my_flag as int
-- than change your statement to simply fill the value of the flag
CASE (SELECT COUNT(1) FROM table_name)
WHEN 1 THEN SET #my_flag = 1
WHEN 0 THEN SET #my_flag = 0
ELSE SET #my_flag = -1
END
IF (NOT #my_flag in (-1, 0))
BEGIN
SET #Item_Num = (SELECT Item_Num FROM table_name) -- consider a filter here
END
ELSE
BEGIN
IF (-1 = #my_flag) RAISERROR('too many records', 0, 0)
IF (0 = #my_flag) RAISERROR('no records', 0, 0)
END
END TRY
BEGIN CATCH
--- rest of the code goes here....

Answer 4

With #TempTable as
(
Select * from ...
-- record set that determines if I should error out
)
SELECT CASE WHEN COUNT(1) > 0
THEN 'Multiple records in database'
ELSE 0
END AS [Value]
FROM #TempTable
datatype mismatch will kill the method if you're trying to error out this whole call with TSQL. Worked in my case because it was a heavy process that I needed to know was transferred correctly. If My record set was >1 then I know I should fail this. Useful if you're using SSIS or multiple methods within a .NET environment