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Editorial Team
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Asked: 2026-05-14T19:13:49+00:00

Microsoft SQL Server 2008 (SP1), getting an unexpected ‘Conversion failed’ error. Not quite sure

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Microsoft SQL Server 2008 (SP1), getting an unexpected ‘Conversion failed’ error.

Not quite sure how to describe this problem, so below is a simple example. The CTE extracts the numeric portion of certain IDs using a search condition to ensure a numeric portion actually exists. The CTE is then used to find the lowest unused sequence number (kind of): 

CREATE TABLE IDs (ID CHAR(3) NOT NULL UNIQUE);

INSERT INTO IDs (ID) VALUES ('A01'), ('A02'), ('A04'), ('ERR');

WITH ValidIDs (ID, seq)
AS 
(
 SELECT ID, CAST(RIGHT(ID, 2) AS INTEGER)
   FROM IDs 
  WHERE ID LIKE 'A[0-9][0-9]'
)
SELECT MIN(V1.seq) + 1 AS next_seq
  FROM ValidIDs AS V1
 WHERE NOT EXISTS (
                   SELECT * 
                     FROM ValidIDs AS V2
                    WHERE V2.seq = V1.seq + 1
                  );

The error is, ‘Conversion failed when converting the varchar value ‘RR’ to data type int.’

I can’t understand why the value ID = 'ERR' should be being considered for conversion because the predicate ID LIKE 'A[0-9][0-9]' should have removed the invalid row from the resultset.

When the base table is substituted with an equivalent CTE the problem goes away i.e. 

WITH IDs (ID)
AS
(
 SELECT 'A01'
 UNION ALL 
 SELECT 'A02'
 UNION ALL 
 SELECT 'A04'
 UNION ALL 
 SELECT 'ERR' 
),
ValidIDs (ID, seq)
AS 
(
 SELECT ID, CAST(RIGHT(ID, 2) AS INTEGER)
   FROM IDs 
  WHERE ID LIKE 'A[0-9][0-9]'
)
SELECT MIN(V1.seq) + 1 AS next_seq
  FROM ValidIDs AS V1
 WHERE NOT EXISTS (
                   SELECT * 
                     FROM ValidIDs AS V2
                    WHERE V2.seq = V1.seq + 1
                  );

Why would a base table cause this error? Is this a known issue?

UPDATE @sgmoore: no, doing the filtering in one CTE and the casting in another CTE still results in the same error e.g. 

WITH FilteredIDs (ID)
AS 
(
 SELECT ID
   FROM IDs 
  WHERE ID LIKE 'A[0-9][0-9]'

), 
ValidIDs (ID, seq)
AS 
(
 SELECT ID, CAST(RIGHT(ID, 2) AS INTEGER)
   FROM FilteredIDs 
)
SELECT MIN(V1.seq) + 1 AS next_seq
  FROM ValidIDs AS V1
 WHERE NOT EXISTS (
                   SELECT * 
                     FROM ValidIDs AS V2
                    WHERE V2.seq = V1.seq + 1
                  );
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1 Answer

  1. Editorial Team

    It’s a bug and has already been reported as SQL Server should not raise illogical errors (as I said, it’s hard to describe this one!) by Erland Sommarskog.

    The response from the SQL Server Programmability Team is, “the issue is that SQL Server raises errors [too] eagerly due to pushing of prediates/expressions during query execution without considering the logical result of the query.”

    I’ve now voted for a fix, everyone do the same please 🙂

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