Tag Archives: Dates

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The query was perfectly broken…

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.

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I won’t be late for this, late for that because I have Time Zone Info….

One of the new items in SQL Server 2016 is the super awesome time_zone_info table.  When I heard about it, I started to think about all the cool things that it could help me do.  First, let’s look at the table.

SELECT *
FROM [sys].[time_zone_info]

time_zone_info

Yes, it is 132 rows of magic! Now that we have this super cool table, how do we use it? Let’s pretend that my data is time-stamped in US Mountain Standard Time, but I want to display it in Western Australia Standard Time.  I would do it like this:

SELECT GETDATE() AS GETDATE_Time,
 GETDATE() AT TIME ZONE 'US Mountain Standard Time' AS Mountain_Time,
 GETDATE() AT TIME ZONE 'US Mountain Standard Time' 
   AT TIME ZONE 'W. Australia Standard Time'AS W_Aus_Time;

I am including the GetDate column so you can see that GetDate is using my time zone, but I have to tell it what time zone it is before I can convert it to another one.

time-zone-query

Caution: If I put in the Hawaiian time zone instead of Mountain time on the GetDate, SQL won’t correct me, it will just do the math like a good little system and assume I know what time zone I am using to start.

I am am really loving the new features in SQL 2016! I hope you are enjoying it too!

This posts song is Cleopatra by The Lumineers

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Yeah, that’s my kind of T-SQL!

I have a Dates table and I really love it. This week I needed to do a calculation where I take my monthly budget, divide it by the number of working days in a month and then times it by the working days for the week. That will give me a weekly budget number without having to store it, plus my company only gives me monthly numbers. I had already figured out the daily number and had the calculation working for a 5 day work week when New Years day made everyone notice that it was 5 and I was cheating. So I added a Column to my Dates table to tell me on any given day the number of working days in that week. My weeks run from Sunday to Saturday. I have a Date in my dates table for both the start of the week and the start of the next week. I have a flag (1,0) that says whether or not a day is a weekday and another for whether a day is a holiday. Using this flags, I can pass in a range of dates and get the number of working weekdays.

UPDATE Dates 
SET WorkDaysInWeek = WeekDays 
FROM Dates D 
INNER JOIN  
(SELECT BegW, SUM(IsWeekDay) - SUM(IsHoliday) AS WeekDays 
FROM Dates 
WHERE FullDate Between BegW AND NextWeekStart 
GROUP BY BegW) W ON D.BegW = W.BegW

—————UPDATE—————

But wait, there’s more. Rob Farley who is on twitter @rob_farley sent me an even better way to do it. It gets rid of the need for a WHERE or GROUP BY because we are using OVER. We hoped it would eliminate the sub-query and it does if our query is only a SELECT, but when I go to do an UPDATE, it says that “Windows functions can only appear in the SELECT and ORDER BY clauses”. Rob suggested I use a CTE instead.  I hadn’t ever used a CTE without joining back to it so he taught me how.

This is the super awesome SELECT:

SELECT BegW,SUM(IsWeekday - IsHoliday) OVER (PARTITION BY BegW) AS WeekDays
FROM Dates

Here is what the UPDATE ended up looking like.

WITH d AS (
SELECT *,SUM(IsWeekday - IsHoliday) OVER (PARTITION BY BegW) AS WeekDays
FROM Dates)
UPDATE d SET WorkDaysInWeek = WeekDays;

The other question that Rob had was what if we have holidays on weekends. This is a great question. At my company, the holiday would be shifted to one of the days of the week, so the counts would still be correct. But if you are in a situation where that is not the case you can change the where clause like this:

WHERE [DayOfWeek] NOT IN (1,7)

As I was writing my where clause, I noticed I did something bad and used a reserved word as a column name. If you happen to make this mistake as well, just make sure to put square brackets [] around the word when you use it.

Huge thank you to Rob for being so kind to help me be better! One of my new goals is to play with OVER and understand how to use it and when. I am also going to be learning more about CTE’s and not having to use joins. Yay! New toys!!!!!