Voice of the DBA

Author: way0utwest

  • T-SQL Tuesday #26 – Second Changes with Date/Time

    TSQL2sDay150x150

    I missed the very first T-SQL Tuesday, so when this month’s topic of second chances came up, I decided to write that one.

    If you are unsure of what T-SQL Tuesday is, follow the link to this month’s topic to get the rules and description and then write a blog post.

    Date/Time Challenges

    I picked an easy one, but one that I continue to see asked in the forums by people new to SQL Server. I suspect we’ll see less questions over time as more people take advantage of the new DATE and TIME datatypes in SQL Server 2008 and later, but maybe not. Lots of people are still sure that they need to keep those items together.

    In any case, have you ever seen sales data like this:

    OrderID     OrderDate               CustomerID  OrderAmount
    ———– ———————– ———– ————
    1           1982-05-19 06:31:48.950 1           579040.5070
    2           1994-11-27 17:14:41.790 2           348808.5860
    3           1972-11-08 17:40:01.170 3           758992.3650
    4           1972-05-31 01:19:05.530 4           779853.1990
    5           1994-12-22 10:40:57.410 5           666173.8040
    6           1974-04-03 01:42:29.490 6           134218.2330
    7           1976-06-22 15:21:18.910 7           322938.6950
    8           1953-08-05 23:00:34.620 8           14169.7580
    9           1971-08-16 22:33:28.970 9           586057.3820
    10          2002-03-28 13:08:00.420 10          632785.0760

    Here’s some DDL, you can create your own data, but here are a few rows:

    CREATE TABLE SalesOrders
( OrderID INT IDENTITY(1,1) , OrderDate DATETIME , CustomerID INT , OrderAmount NUMERIC(10, 4) ) INSERT SalesOrders (OrderDate, CustomerID, OrderAmount) VALUES ( '1982-05-19 06:31:48.950', 1, 579040.5070), ( '1994-11-27 17:14:41.790', 2, 348808.5860), ( '1972-11-08 17:40:01.170', 3, 758992.3650), ( '1972-05-31 01:19:05.530', 4, 779853.1990), ( '1994-12-22 10:40:57.410', 5, 666173.8040)

    If I want to get all the sales in May of 1972, I get query them all like this:

    SELECT OrderDate
, OrderAmount
 FROM SalesOrders
 WHERE OrderDate > '1972/5/1' AND OrderDate <= '1972/5/31'

    I get 4 rows back. That’s something people often write when they get input from a user. A user has a start and end edit box, they enter “1972/5/1’” in the start box (or use a calendar picker) and then enter “1972/5/31” in the other. I’m using ISO dates to make this clear, though in the US it would normally display like “5/1/1972” and in the UK as “1/5/1972”.

    However, that isn’t quite correct. If I run this query:

    SELECT OrderDate
, OrderAmount
 FROM SalesOrders
 WHERE MONTH(OrderDate) = 5
  AND YEAR(OrderDate) = 1972

    I actually get 6 rows. The data rows for May 1972 are:

    OrderDate               OrderAmount

    ———————– —————————-

    1972-05-31 01:19:05.530 779853.1990

    1972-05-13 09:26:52.590 676848.9700

    1972-05-28 21:05:28.840 923425.0510

    1972-05-07 10:59:09.930 266079.6480

    1972-05-01 04:21:01.250 464241.6480

    1972-05-31 01:19:05.530 779853.1990

    What’s happening?

    If you look at the OrderDates for May 31, you see two values that have a time of 1:19:05am. Those are excluded from the query, which has an end date of “1972/5/31”. Why? This query:

    SELECT CAST( '1972/5/31' AS DATETIME)

    shows why. It returns:

    1972-05-31 00:00:00.000

    That’s midnight between the 30th and 31st, which is before 1:19:05am. When a datetime value is converted in SQL Server, without a time component, it defaults to the beginning of the day. That works great for the start date, not so good for the end date.

    Fixing this

    There are two real fixes here. Well, maybe more. You can query on the month and year, but those functions can disrupt indexes, so I don’t recommend them. The two main fixes are:

    • add a time component
    • add a day

    The first fix is the addition of the last time of the day to your query.

    SELECT OrderDate
, OrderAmount
 FROM SalesOrders
 WHERE OrderDate > '1972/5/1' AND OrderDate <= '1972/5/31 23:59:59.997PM'

    In some type of code, it looks more like this:

    DECLARE @end DATETIME SELECT @end = '1972/5/31' SELECT @end = @end + '23:59:59.997' SELECT OrderDate
, OrderAmount
 FROM SalesOrders
 WHERE OrderDate > '1972/5/1' AND OrderDate <= @end

    Assume the first select is actually coming from the user.

    The second fix is to add a day, and actually query like this, which is what I’d recommend:

     SELECT OrderDate
, OrderAmount
 FROM SalesOrders
 WHERE OrderDate > '1972/5/1' AND OrderDate < '1972/6/1'

    In this case, instead of querying for May 31, we move to June 1 (the next day) and then change the <= to a < only. This gets all orders occurring up until the end of May 31, but before June 1.

    Easy fixes, but so often there’s code that doesn’t allow for the time component. Take a minute and check your reports, and be sure you aren’t underreporting any data to your clients or customers.

    Also be sure to check out the new datatypes in SQL Server 2008 and later:

  • Referencing Remote Data

    Maybe if I searched more, I'd use synonyms

    One of the features added to SQL Server a few years ago were the ability to create synonyms and use those to reference other objects. The ability to create synonyms is something that I had wanted for years in SQL Server, but when they were released, I found them to be a tool that I rarely reached for. Whether it was because this was something I rarely needed to accomplish, or because my habits were too ingrained, I’m not sure, but I have only created synonyms for testing purposes and not for use in any production databases.

    When a developer needs to reference data in another database (or on another server), they have a variety of ways in which they can do this. Some people prefer a three or four part naming convention, others use a view local to the database, and still others might use synonyms. While they all work, from a maintenance standpoint, I think a view or synonym provide a nice layer of abstraction while minimizing the potential maintenance headaches of future changes.

    If you find synonyms more useful than local views, I would be interested in knowing why. They seem to almost operate in the same way to me, but for some reason I find views to be easier to track and manage. Perhaps it’s just an ingrained habit from years of making do with views, or maybe it’s my habit of browsing for objects, instead of using a tool like SQL Search.

    Whichever method you use, I do urge you to always consider a layer of abstraction. That other database you reference might be located on the same instance today, but in the future it might grow and require it’s own instance. If you have three part naming buried in all of your stored procedure or application code, it might not be as simple as a global search and replace to make changes. If it’s not, then you are wasting development time down the road by not having a layer of abstraction implemented at the beginning.

    Steve Jones

    Steve Jones

    The Voice of the DBA Podcasts

  • DBAs in Denver

    I have a friend looking for a production DBA in Denver, so if you’re interested, email me at SQLServerCentral.

    They need a production DBA in the South Denver area, backups, config, query tuning, replication, partitioning. SQL Server 2005-2008 R2.

  • Congrats, Yanni

    Sunday was an exciting morning for me and quite a few others in the SQL Server community. My morning was interrupted constantly as I was tied to my Twitter stream, looking for updates like this one:

    One of the updates I caught during my morning

    I had puppy duty, dinner to cook that required a trip to the store and 8 hours of cooking. I had a few chores to handle like horses to feed, and I found myself taking longer to do those things than I should have because I was constantly checking my phone. I wasn’t the only one as I saw 6 or8 other people in the SQL community, all cheering Yanni on. I’m sure there were others outside my twitter feed as well.

    We were all sending tweets that the Runmeter app will read to you over headphones. I’ve used the app and it’s cool when someone sends you a note when you’re running, even if it interrupts the music.

    I probably sent 20-30 tweets to try and help cheer Yanni on during her first marathon. I met Yanni last year, and I’ve been following her journey as she climbed a mountain, ran a half marathon (with other SQL people), and made the decision to run just a marathon, but a marathon and a half in a week. As someone that’s run a marathon, I can appreciate her effort, accomplishment, and I’m impressed.

    I’m also thrilled to be a part of the SQL Server community, which in many ways seems to be a tighter set of groups of people. Groups because we aren’t one big happy family, but we are a series of smaller ones that support each other, cheer each other one, defend each other, and laugh with each other. The biggest reason that I attend events is to meet people and spend time with them.

    Yanni is never without a smile on her face, and I’m sure part of it is the wonderful people who work with SQL Server in their spare time, when they’re not making life a little bit more interesting to each other, 140 characters at a time.

    I congratulate Yanni, and I urge you to do the same, with a comment on her marathon post.

    Update: This is the post race post.