Arithmatic conversion error in query

Question

I have a stored procedure for a report . The underlying table has a decimal(18,2) column.
In the Stored Procedure code, the conversion to decimal (12,2) gives an error as one of the records has a value that exceeds (12,2).
I am running this report for a userID that does not have permission to this project (so bad row will not be included in the result).
When running the Stored procedure from query analyser, this row with bad data is not selected based on #TblPermission and SP returns other rows without errors.
The issue is when running the SP from the application, i am getting an arithmatic conversion error.
It appears that the SELECT conversions are happening even before join on the temp table is satisfied.
Shouldnt it filter data based on the join first and then do the conversions (like when run from query analyser) ?
We will be fixing the conversion issue at a later point.
But any ideas/suggestions on how to address this to stop it from erroring when called from the UI ?
CREATE PROCEDURE [dbo].[Test_Report]
@_user varchar(10)
AS
BEGIN
SELECT PID INTO #TblPermission FROM dbo.tbl_permissions(nolock) WHERE userid= @_user
SELECT A.NAME,
 CAST(A.ProjectAmount as decimal(12,2))  Amt -- giving an arithmatic conversion error
FROM TableA A
Join #TblPermission UP on UP.PID= A.PID
WHERE A.ProjectType ='mapped'
end

Answer 1

It appears that the SELECT conversions are happening even before join on the temp table is satisfied.
Shouldnt it filter data based on the join first and then do the conversions (like when run from query analyser) ?
The answer to that question is no.  SQL is allowed to process a query in whatever order it believes will be most efficient.  So it can do the conversion, and later do the WHERE clause.  So you have to write the code so that it is safe if SQL
does that.  One way is to use a case statement to only do the conversion when the value will fit in decimal(12,2), so you could do
SELECT A.NAME,
Case When A.ProjectAmount Between -9999999999.99 And 9999999999.99 Then CAST(A.ProjectAmount as decimal(12,2)) End Amt
FROM TableA A
Join #TblPermission UP on UP.PID= A.PID
WHERE A.ProjectType ='mapped'
That will work on all releases of SQL.
If you are on SQL 2012 or later, you can use the new TRY_CONVERT function which will do conversions without causing errors.  Instead of an error when the valued being converted is invalid, you get NULL.  So that would be
SELECT A.NAME,
TRY_CONVERT(decimal(12,2), A.ProjectAmount) Amt
FROM TableA A
Join #TblPermission UP on UP.PID= A.PID
WHERE A.ProjectType ='mapped'
Tom