Using DATEDIFF in T-SQL

Question

I am using DATEDIFF in an SQL statement. I am selecting it, and I need to use it in WHERE clause as well. This statement does not work...

SELECT DATEDIFF(ss, BegTime, EndTime) AS InitialSave
FROM MyTable
WHERE InitialSave <= 10

It gives the message: Invalid column name "InitialSave"

But this statement works fine...

SELECT DATEDIFF(ss, BegTime, EndTime) AS InitialSave
FROM MyTable
WHERE DATEDIFF(ss, BegTime, EndTime) <= 10

The programmer in me says that this is inefficient (seems like I am calling the function twice).

So two questions. Why doesn't the first statement work? Is it inefficient to do it using the second statement?

Answer 1

Note: When I originally wrote this answer I said that an index on one of the columns could create a query that performs better than other answers (and mentioned Dan Fuller's). However, I was not thinking 100% correctly. The fact is, without a computed column or indexed (materialized) view, a full table scan is going to be required, because the two date columns being compared are from the same table!

I believe there is still value in the information below, namely 1) the possibility of improved performance in the right situation, as when the comparison is between columns from different tables, and 2) promoting the habit in SQL developers of following best practice and reshaping their thinking in the right direction.

Making Conditions Sargable

The best practice I'm referring to is one of moving one column to be alone on one side of the comparison operator, like so:

SELECT InitialSave = DateDiff(second, T.BegTime, T.EndTime)
FROM dbo.MyTable T
WHERE T.EndTime <= T.BegTime + '00:00:10'

As I said, this will not avoid a scan on a single table, however, in a situation like this it could make a huge difference:

SELECT InitialSave = DateDiff(second, T.BegTime, T.EndTime)
FROM
   dbo.BeginTime B
   INNER JOIN dbo.EndTime E
      ON B.BeginTime <= E.EndTime
      AND B.BeginTime + '00:00:10' > E.EndTime

EndTime is in both conditions now alone on one side of the comparison. Assuming that the BeginTime table has many fewer rows, and the EndTime table has an index on column EndTime, this will perform far, far better than anything using DateDiff(second, B.BeginTime, E.EndTime). It is now sargable, which means there is a valid "search argument"--so as the engine scans the BeginTime table, it can seek into the EndTime table. Careful selection of which column is by itself on one side of the operator is required--it can be worth experimenting by putting BeginTime by itself by doing some algebra to switch to AND B.BeginTime > E.EndTime - '00:00:10'

Precision of DateDiff

I should also point out that DateDiff does not return elapsed time, but instead counts the number of boundaries crossed. If a call to DateDiff using seconds returns 1, this could mean 3 ms elapsed time, or it could mean 1997 ms! This is essentially a precision of +- 1 time units. For the better precision of +- 1/2 time unit, you would want the following query comparing 0 to EndTime - BegTime:

SELECT DateDiff(second, 0, EndTime - BegTime) AS InitialSave
FROM MyTable
WHERE EndTime <= BegTime + '00:00:10'

This now has a maximum rounding error of only one second total, not two (in effect, a floor() operation). Note that you can only subtract the datetime data type--to subtract a date or a time value you would have to convert to datetime or use other methods to get the better precision (a whole lot of DateAdd, DateDiff and possibly other junk, or perhaps using a higher precision time unit and dividing).

This principle is especially important when counting larger units such as hours, days, or months. A DateDiff of 1 month could be 62 days apart (think July 1, 2013 - Aug 31 2013)!

Answer 2

You can't access columns defined in the select statement in the where statement, because they're not generated until after the where has executed.

You can do this however

select InitialSave from 
(SELECT DATEDIFF(ss, BegTime, EndTime) AS InitialSave
FROM MyTable) aTable
WHERE InitialSave <= 10

As a sidenote - this essentially moves the DATEDIFF into the where statement in terms of where it's first defined. Using functions on columns in where statements causes indexes to not be used as efficiently and should be avoided if possible, however if you've got to use datediff then you've got to do it!