Thursday, December 15, 2011

TSQL SELECT Convert or Cast DateTime but WHERE fails

Can a select return when a where fails?

This zen like question came up this afternoon while digging through some rather raw varchar table data. The answer is yes, and part of answer is coming up with the right question. While googling around I ended up settling on "CONVERT fails in where clause but not in select".

Consider the following problem:
SELECT * FROM dbo.MyTable
WHERE ISDATE(value)=1 AND AND  CAST(Value AS datetime) > GETUTCDATE()


SELECT * FROM dbo.MyTable
WHERE ISDATE(value)=1 AND CONVERT(datetime,Value) > GETUTCDATE()

2012-01-02 00:00:00.000


SELECT CONVERT(datetime,Value) FROM dbo.MyTable


SELECT CAST(Value AS datetime) FROM dbo.MyTable

Msg 241, Level 16, State 1, Line 1
Conversion failed when converting date and/or time from character string.

To paraphrase the above, SQL Server may evaluate rows outside of the expected WHERE clause based on how the optimizer decides to limit the result set. This left me with three solutions:

  1. Create an index to persuade the optimizer to avoid the plan that evaluates non-date columns... perhaps not.

  2. Reload cast or converted data into a #temporary table, yes this will work but really?

  3. My solution below compliments of the path of least resistance, add some case logic around the value column
SELECT Value FROM dbo.MyTable
WHERE CASE ISDATE(value)=1 THEN CONVERT(datetime,Value)
--OIC ~(:o)

When you think about how SQL Server has to discover the table data it makes sense. If you haven't run into the condition before it may cause a little head scratching. The better solution would be to use a date time column in the first place if possible, but hopefully with this post and the corresponding stackoverflow post you can save a few extra hairs on your head.