I have been teaching a T-SQL 101 class and for the homework, we asked the students to get all the records where our heroes had a birthdate between 1995 through 1999. I expected something like this:
SELECT FirstName, LastName, Birthdate FROM Heroes WHERE Birthdate BETWEEN '1/1/1995' AND '12/31/1999' OR SELECT FirstName, LastName, Birthdate FROM Heroes WHERE Birthdate >= '1/1/1995' AND Birthdate <= '12/31/1999'
Imagine my surprise when one of the students turned in this:
SELECT FirstName, LastName, Birthdate FROM Heroes WHERE Birthdate BETWEEN '1995' AND '1999'
When I first saw the query I thought, “There is no way they ran that and it worked.” So I wrote it up and ran it on my data. Guess what? IT RUNS AND RETURNS DATA! I was shocked.
I started looking at the plan and what it did to the data and found that it had done an implicit conversion on the dates and assumed 1/1/1995 to 1/1/1999 based on the year. So we were missing data from the results, but I was still in shock that it had run in the first place and shared this information with my co-worker who reminded me that dates get crazy and if I only put in ’12/31/1999′ and there is a time in the field, it will cut off most of the times within that day because it will assume I want ’12/31/1999 00:00:00′. If I want the full day, I need to get in the habit of specifying ’12/31/1999 23:59:59′ or ‘1/1/2000 00:00:00’ and then test my results to make sure I am getting what I truly want back from the database.
The song for this post is BANNERS – Perfectly Broken.