confusing behavior with case and isnumeric - sql-server

Question

In my stored procedure I have a case statement that is supposed to test whether a field is numeric and handle that but I get the same error no matter what. I have tried below:
CASE WHEN ISNUMERIC(subScores.ScoreValue) = 1 THEN CAST(subScores.ScoreValue AS REAL) ELSE -1 END
I also used a custom convert function because I have read that isnumeric isn't 100% reliable but that received the same error. I also tried using CONVERT but no success there. I even replaced the isnumeric with 0=1 which should always fail but I still get the same error which I find really strange. This is my error:
Error converting data type nvarchar to real
The values that it is checking look like:
A0, A10, A23, B9, B51, C90, C100, etc
Any ideas for this behavior?
Here is the full procedure. The issue happening on the first case in the where clause after the THEN instead of using -1 its using the actual field. It also contain my usage of ISNUMERIC and my custom function:
#strDemoCodeIds nvarchar(100),
#strYearIds nvarchar(100),
#intRosterSetId int,
#intSchoolId int,
#intTeachId int,
#intGradeId int,
#intDeptId int,
#intCourseId int,
#intPeriodId int,
#strTestInstIds nvarchar(100),
#intTestTypeId int,
#intSubjectId int,
#intScoreTypeId int,
#strSuppScoreTypeId nvarchar(20),
#intClusterScoreTypeId int,
#intClusterColorScoreTypeId int,
#intLocalReportId int,
#strSortMethod nvarchar(20),
#intAnchorYear int
AS
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;
DECLARE #topYearID INT
DECLARE #subs TABLE (subId int)
IF #intSubjectId < 100
BEGIN
INSERT INTO #subs
SELECT MM_Test_Subjects.pkTestSubjectID
FROM MM_Test_Subjects
WHERE MM_Test_Subjects.fkCSTStrandID = #intSubjectId
END
ELSE
BEGIN
INSERT INTO #subs VALUES (#intSubjectId)
END
DECLARE #years TABLE (yearId int)
INSERT INTO #years
SELECT number FROM itot(#strYearIds, N',')
SET #topYearID = #intAnchorYear
SELECT s.pkStudentID,
s.LastName,
s.FirstName,
s.StudentNumber,
st.fkTestInstanceID,
bands.Description AS 'PL',
bands.Color,
subScoresSupp.scoreValue AS 'SuppScore',
clusters.pkTestClusterID,
clusterScores.ScoreValue,
colors.ScoreValue,
ti.TestInstanceName,
tp.TestPeriodTitle,
subScores.ScoreValue AS 'PLScore',
ScoreType.pkScoreTypeID,
bands.StackPosition AS 'StackPosition'
FROM Students s
INNER JOIN StudentTests st ON s.pkStudentID = st.fkStudentID
INNER JOIN StudentTests baseSt ON s.pkStudentID = baseSt.fkStudentID
AND baseSt.fkGradeID = CASE #intGradeId WHEN 99 THEN baseSt.fkGradeID ELSE #intGradeId END
AND EXISTS (SELECT * FROM StudentScores_Subject baseSubScores WHERE baseSubScores.fkStudentTestID = baseSt.pkStudentTestID
AND baseSubScores.fkTest_SubjectID IN (SELECT subId FROM #subs)
AND baseSubScores.fkScoreTypeID = #intScoreTypeId)
INNER JOIN StudentTestDemographics d ON d.fkStudentTestID = baseSt.pkStudentTestID
INNER JOIN itot(#strDemoCodeIds, N',') tblDemoCodes ON d.fkDemographicCodeID = CASE #strDemoCodeIds WHEN '0' THEN d.fkDemographicCodeID ELSE tblDemoCodes.number END
INNER JOIN StudentScores_Subject subScores ON subScores.fkStudentTestID = st.pkStudentTestID
AND subScores.fkTest_SubjectID IN (SELECT subId FROM #subs)
AND subScores.fkScoreTypeID = #intScoreTypeId
LEFT JOIN PerformanceLevelReportBands bands ON bands.fkPerformanceLevelReportID = #intLocalReportId
LEFT JOIN MMARS_Web_TestInfo_California.dbo.PerfLevelReportBandCutScores cutScores ON cutScores.fkPerformanceLevelReportBandID = bands.pkPerformanceLevelReportBandID
AND cutScores.fkGradeID = #intGradeId
AND cutScores.fkTestSubjectID IN (SELECT subId FROM #subs)
RIGHT JOIN PerfLevelReportBandComponents bandComponents ON bandComponents.fkPerformanceLevelReportBandID = bands.pkPerformanceLevelReportBandID
LEFT JOIN StudentScores_Subject subScoresSupp ON subScoresSupp.fkStudentTestID = st.pkStudentTestID
AND subScoresSupp.fkTest_SubjectID IN (SELECT subId FROM #subs)
AND subScoresSupp.fkScoreTypeID IN (SELECT tblsubScoresSupp.number FROM itot(#strSuppScoreTypeId, N',') tblsubScoresSupp)
LEFT JOIN MM_ScoreTypes ScoreType ON ScoreType.pkScoreTypeID = subScoresSupp.fkScoreTypeID
LEFT JOIN StudentScores_Cluster clusterScores ON clusterScores.fkStudentTestID = st.pkStudentTestID
AND clusterScores.fkTest_SubjectID IN (SELECT subId FROM #subs)
AND clusterScores.fkScoreTypeID = #intClusterScoreTypeId
LEFT JOIN StudentScores_Cluster colors ON colors.fkStudentTestID = st.pkStudentTestID
AND colors.fkTest_SubjectID IN (SELECT subId FROM #subs)
AND colors.fkScoreTypeID = #intClusterColorScoreTypeId
AND colors.fkTest_ClusterID = clusterScores.fkTest_ClusterID
LEFT JOIN MM_Test_Clusters clusters ON clusters.pkTestClusterID = clusterScores.fkTest_ClusterID
INNER JOIN TestInstances ti ON ti.pkTestInstanceID = st.fkTestInstanceID
INNER JOIN TestInstances baseTi ON baseTi.pkTestInstanceID = baseSt.fkTestInstanceID
AND baseTi.fkSchoolYearID = #topYearID
INNER JOIN CAHSEE_TestPeriods tp on tp.pkTestPeriodID = ti.fkTestPeriodID
INNER JOIN CAHSEE_TestPeriods baseTp on baseTp.pkTestPeriodID = baseTi.fkTestPeriodID
INNER JOIN Roster_Students rs ON rs.fkStudentID = s.pkStudentID
INNER JOIN Roster_Groups rg ON rg.fkTestInstanceID = baseTi.pkTestInstanceID
AND ti.pkTestInstanceID IN (SELECT tblTestInstances.number FROM itot(#strTestInstIds, N',') tblTestInstances)
AND baseTi.pkTestInstanceID IN (SELECT tblTestInstances.number FROM itot(#strTestInstIds, N',') tblTestInstances)
AND rg.fkRosterSetID = #intRosterSetId
AND rg.pkRosterGroupID = rs.fkRosterGroupID
AND (rg.fkSchoolID = #intSchoolId OR #intSchoolId = 0)
AND (rg.fkTeacherID = #intTeachId OR #intTeachId = 0)
AND (rg.fkGradeID = baseSt.fkGradeID)
AND (rg.fkDepartmentID = #intDeptId OR #intDeptId = 0)
AND (rg.fkCourseID = #intCourseId OR #intCourseId = 0)
AND (rg.fkPeriodID = #intPeriodId OR #intPeriodId = 0)
WHERE (bandComponents.ScoreValue = subScores.ScoreValue)
OR
((CASE WHEN ISNUMERIC(subScores.ScoreValue) = 1 THEN CAST(subScores.ScoreValue AS REAL) ELSE subScores.ScoreValue END)
BETWEEN (CASE WHEN dbo.TryConvertInt(bandComponents.minScore) = 1 THEN CAST(bandComponents.minScore AS REAL) ELSE bandComponents.minScore END) and
(CASE WHEN dbo.TryConvertInt(bandComponents.maxScore) = 1 THEN CAST(bandComponents.maxScore AS REAL) ELSE bandComponents.maxScore END)/*CAST(ISNULL(bandComponents.maxScore,'0') AS INT)*/)
OR
((CASE WHEN ISNUMERIC(subScores.ScoreValue) = 1 THEN CAST(subScores.ScoreValue AS REAL) ELSE -1 END)
BETWEEN ISNULL(cutScores.minScore,0) and ISNULL(cutScores.maxScore,0))
GROUP BY s.pkStudentID, s.LastName, s.FirstName, s.StudentNumber, st.fkTestInstanceID, bands.Description, bands.Color, subScoresSupp.scoreValue,
clusters.pkTestClusterID, clusterScores.ScoreValue, colors.ScoreValue, ti.TestInstanceName, tp.TestPeriodTitle, subScores.ScoreValue,
ScoreType.pkScoreTypeID, bands.StackPosition
ORDER BY
CASE WHEN #strSortMethod = 'ScoreDesc'
THEN bands.StackPosition END DESC,
CASE WHEN #strSortMethod = 'ScoreAsc'
THEN bands.StackPosition END ASC,
CASE WHEN #strSortMethod = 'ScoreDesc' AND ISNUMERIC(subScores.ScoreValue) = 1
THEN CONVERT(int, subScores.ScoreValue) END DESC,
CASE WHEN #strSortMethod = 'ScoreAsc' AND ISNUMERIC(subScores.ScoreValue) = 1
THEN CONVERT(int, subScores.ScoreValue) END ASC,
s.LastName, s.FirstName, s.pkStudentID, st.fkTestInstanceID
END

Answer 1

The problem is that the case statement is trying to return only one type of value so if the THEN is trying to convert to real and return that back then the ELSE is going to try and do the same so even if there is no cast it is going to try to convert it implicitly. This explains why this works:
DECLARE #scoreValue nvarchar(10)
SET #scoreValue = N'A53'
SELECT CASE WHEN ISNUMERIC(#scoreValue) = 1 THEN CAST(#scoreValue AS REAL) ELSE 1.0 END
But this does not:
DECLARE #scoreValue nvarchar(10)
SET #scoreValue = N'A53'
SELECT CASE WHEN ISNUMERIC(#scoreValue) = 1 THEN CAST(#scoreValue AS REAL) ELSE #scoreValue END
So the key is to use sql_variant so this works:
CASE WHEN ISNUMERIC(#scoreValue) = 1 THEN CAST(#scoreValue AS REAL) ELSE CAST(#scoreValue AS sql_variant) END

Answer 2

I will try for your stuff then find this iss
select CASE WHEN ISNUMERIC(CONVERT(nvarchar,'- 5')) = 1 THEN CAST(CONVERT(nvarchar,'- 5') AS REAL) ELSE -1 END
Isnumeric function should be return of this value true, and convert function should not convert value into real because space in value.
find data of this type in your column like scorevalue.

Answer 3

The problem is not the line you are pointing !
This line is ok :
CASE WHEN ISNUMERIC(subScores.ScoreValue) = 1 THEN CAST(subScores.ScoreValue AS REAL) ELSE -1 END
But in the first case, just after the WHERE clause you have this line
CASE WHEN ISNUMERIC(subScores.ScoreValue) = 1 THEN CAST(subScores.ScoreValue AS REAL) ELSE subScores.ScoreValue END
It cannot work as if ScoreValue is not numeric, you have a case with real datas and varchar data (I guess ScoreValue is varchar)
You have to put once again -1 instead of ScoreValue to make it work

Answer 4

/*Show how isnumeric differs from TRY_CONVERT*/
declare #temp table (dec int, chr char(1) , isnum int,tryconvert int)
declare #loop int = 1
declare #i int
while #loop < 127
begin
set #i = #loop
insert into #temp (dec,chr,isnum,tryconvert)
select #i,char(#i) charac, isnumeric(char(#i)), TRY_CONVERT(int,char(#i))
set #loop = #loop + 1
end
select * from #temp where isnum = 1