T-SQL

The date time data type and the date time functions within SQL are things that I see coming up time and time again in news groups and forums. Questions on how to get rid of the time, how to get the first day of the week, the last day of the month and so on. With the new Date and Time data types coming in SQL 2008, things will get easier, nut until then we have to do things the hard way.

In systems I’ve worked on I’ve seen several implementations of functions to find the first and last day of a week, a month or a quarter. Some have worked well, some have worked and others, well, haven’t

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In Part 1 I discussed why some queries that look like they should work fail with a conversion error. In this part, I’m going to go over some solutions to the problem.

1. Probably best solution, at least in the long term is, as I mentioned previously, to correct the data type of the column and to remove the data that doesn’t fit. This is probably, however, the hardest of the solutions to implement, due to the potential impact.

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Or “I’m filtering on ISDATE(dt) = 1. Why am I still getting conversion errors?”

Well, to put it simply, because SQL’s evaluating the where clause in a different order to what you may have expected.

Let’s try a simple example.

SELECT * FROM tbl_TestingConversion
WHERE ISDate(TheDate)=1
 AND CAST(TheDate AS DATETIME) > '2007/07/01'

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