SQL invalid conversion return null instead of throwing error

Question

I have a table with a varchar column, and I want to find values that match a certain number. So lets say that column contains the following entries (except with millions of rows in real life):

123456789012
2345678
3456
23 45
713?2
00123456789012

So I decide I want all the rows which are numerically 123456789012 write a statement that looks something like this:

SELECT * FROM MyTable WHERE CAST(MyColumn as bigint) = 123456789012

It should return the first and last row, but instead the whole query blows up because it can't convert the "23 45" and "713?2" to bigint.

Is there another way to do the conversion that will return NULL for values that can't convert?

Answer 1

SQL Server does NOT guarantee boolean operator short-circuit, see On SQL Server boolean operator short-circuit. So all solution using ISNUMERIC(...) AND CAST(...) are fundamentally flawed (they may work, but hey can arbitrarily fail later dependiong on the generated plan). A better solution is using CASE, as Thomas suggests: CASE ISNUMERIC(...) WHEN 1 THEN CAST(...) ELSE NULL END. But, as gbn pointed out, ISNUMERIC is notoriously finicky in identifying what 'numeric' means and many cases where one would expect it to return 0 it returns 1. So mixing the CASE with the LIKE:

CASE WHEN MyRow NOT LIKE '%[^0-9]%' THEN CAST(MyRow as bigint) ELSE NULL END

But the real problem is that if you have millions of rows and you have to search them like this, you'll always end up scanning end-to-end since the expression is not SARG-able (no matter how we rewrite it). The real issue here is data purity, and should be addressed at the appropriate level, where the data is populated. Another thing to consider is if is possible to create a persisted computed column with this expression and create a filtered index on it which eliminates NULL (ie. non-numeric). That would speed up things a little.

Answer 2

If you are using SQL Server 2012 you can use the 2 new methods:

• TRY_CAST()
• TRY_CONVERT()

Both methods are equivalent. They return a value cast to the specified data type if the cast succeeds; otherwise, returns null. The only difference is that CONVERT is SQL Server specific, CAST is ANSI. using CAST will make your code more portable (although not sure if any other database provider implements TRY_CAST)

Answer 3

ISNUMERIC will accept empty string and values like 1.23 or 5E-04 so could be unreliable.

And you don't know what order things will be evaluated in so it could still fail (SQL is declarative, not procedural, so the WHERE clause probably won't be evaluated left to right)

So:

• you want to accept value that consist only of the characters 0-9
• you need to materialise the "number" filter so it's applied before CAST

Something like:

SELECT 
*
FROM
   (
   SELECT TOP 2000000000 *
   FROM MyTable
   WHERE MyColumn NOT LIKE '%[^0-9]%'  --double negative rejects anything except 0-9
   ORDER BY MyColumn 
   ) foo
WHERE 
    CAST(MyColumn as bigint) = 123456789012 --applied after number check

Edit: quick example that fails.

CREATE TABLE #foo (bigintstring varchar(100))
INSERT #foo (bigintstring )VALUES ('1.23')
INSERT #foo (bigintstring )VALUES ('1 23')
INSERT #foo (bigintstring )VALUES ('123')

SELECT * FROM #foo
WHERE
   ISNUMERIC(bigintstring) = 1 
   AND 
   CAST(bigintstring AS bigint) = 123