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SQL Server 2008 : T-SQL Tips and Tricks (part 1) - Date Calculations & Sorting Results with the GROUPING Function

5/17/2011 3:55:19 PM
The following sections provide some general tips and tricks to help you get the most from your T-SQL code. Date Calculations Occasionally, you may find that you need to start with a date value and use it to calculate some other date. For example, your SQL code might need to determine what date is the first day of the month or last day of the month. As you may know, working with the datetime data type in SQL Server can be a bit of a challenge. You probably already know how to use the datepart() function to extract specific components of a date (for example, year, month, day). You can then use those components along with a number of functions to calculate a date that you might need. This section provides some examples of algorithms you can use to generate some commonly needed date values. The DATEDIFF function calculates the difference between two dates, where the difference is based on an interval, such as hours, days, weeks, months, years, and so on. The DATEADD function calculates a date by adding an interval of time to a date. In this case, the intervals of time are the same as those used by the DATEDIFFDATEADD and DATEDIFF functions to calculate specific dates requires thinking outside the box a bit to convert the date value into a date you need. You need to start thinking in terms of date intervals—for example, how many date intervals it is from the current date to the date you want to calculate, or how many date intervals it is from today to some other date, such as "2000-01-01", and so on. You use the DATEADD and DATEDIFF functions to calculate your desired date by determining the appropriate date intervals from the current date and then adding or subtracting intervals to arrive at the desired calculated date. Understanding how to use the various date intervals helps you more easily understand how to calculate the desired dates. function. Using the Calculating the First Day of Month Let’s look at a method for determining the first day of the month for a given date. To do this, you start out with the initial date. (In this example, you can use getdate()"1/1/1900". to work with the current system date and time.) The next step is to figure out the number of months between the given date and the date Note The value “1/1/1900” is the default for a date if an empty string ('') is used to represent a date. You can use the DATEDIFF function to determine the number of months from "1/1/1900": `select DATEDIFF(mm,'',getdate())go-----------1321` Now, using the number of months, you can add that result to "1/1/1900" to obtain the first day of the month for the given date: `select DATEADD(mm, DATEDIFF(mm,'',getdate()), '')` By adding the number of months between the given date and "1/1/1900" to "1/1/1900", you are able to arrive at the first day of the current month. In addition, the time portion of the calculated date is set to "00:00:00.000". This technique for calculating a date interval between the current date and the year, "1900-01-01", and then adding the calculated number of intervals to "1900-01-01" can be used to calculate many different dates. The next four examples use the same technique to generate different dates based on the current date. Calculating the First Day of the Year You can use the year interval (yy) to display the first day of the year: `select DATEADD(yy, DATEDIFF(yy,'',getdate()), '')` Calculating the First Day of the Quarter To calculate the first day of the current quarter, you use the quarter (qq) interval: `select DATEADD(qq, DATEDIFF(qq,'',getdate()), '')` Calculating Midnight for the Current Day If you need to truncate the time portion for a datetime value so it reflects the current date at midnight, you can use the date interval (dd) to get the midnight time stamp for the desired date: `select DATEADD(dd, DATEDIFF(dd,'',getdate()), '')` Calculating Monday of the Current Week You can use the week interval (wk) to calculate what date is Monday of the current week: `select DATEADD(wk, DATEDIFF(wk,'',getdate()), '')` Calculating Other Dates As you have seen, by using simple DATEADD and DATEDIFF calculations, you can come up with many different dates that might be valuable. All the examples so far have only calculated the number of date intervals between the current date and "1/1/1900" and then added the appropriate number of intervals to "1900-01-01" to arrive at the calculated date. If you have to calculate other date values, you can use this calculation as the basis and then add or subtract additional intervals to come up with other useful dates. For example, to calculate the last day of the previous month for a given date, you can use the following calculation to determine the first day of the current month and subtract a day from it: `select DATEADD(dd, -1, DATEADD(mm, DATEDIFF(mm,'',getdate()), ''))` You can perform a similar calculation to determine the last day of the previous year, based on the formula to calculate the first date of the current year for the given date: `select DATEADD(dd, -1, DATEADD(yy, DATEDIFF(yy,'',getdate()), ''))` What if you need to determine the last day of the current month for a given date? One way to do this is to calculate the first date of the next month and subtract one day from that. To calculate the first day of the next month, you can use the formula to calculate the first day of the current month and add one to the number of intervals returned by DATEDIFF when comparing the given date to "1/1/1900" to get the first day of the next month: `select DATEADD(mm, DATEDIFF(mm,'',getdate()) + 1, '')` Now that you have the first date of the next month, you simply subtract one day from it to get the last day of the current month: `select DATEADD(dd, -1, DATEADD(mm, DATEDIFF(mm,'',getdate()) + 1, ''))` Similarly, you can modify the formula to calculate the first day of the year to return the last day of the previous year: `select DATEADD(dd, -1, DATEADD(yy, DATEDIFF(yy,'',getdate()) + 1, ''))` Now, let’s try a little more advanced calculation: the first Monday of the current month. To find this, you start with the calculation for the Monday of the current week and modify it slightly. Rather than use getdate() as the date value, you use the calculation to get the first day of the month and add five days to it. Adding five days to the first day of the month ensures that you are in the first full week of the month (SQL Server treats Sunday as the first day of the week, so if the first day of the month was on a Monday, adding 5 days keeps you in the same week. If the first day is Tuesday or later, adding 5 days puts you into the next week). You can use the following calculation to get the first day of the month and add five days to it: `select DATEADD(dd, 5, DATEADD(mm, DATEDIFF(mm,'',getdate()), ''))` Now, you use this expression in place of the getdate() function in the calculation to get the date for Monday of the current week: `select DATEADD(wk, DATEDIFF(wk,'', DATEADD(dd, 5, DATEADD(mm, DATEDIFF(mm,'',getdate()), ''))), '') ` Tip If you find yourself using any of these date calculations frequently, it might be a good idea to create one or more user-defined functions to encapsulate these calculations. It would save your having to reenter the sometimes complex formulas, which can be easily mistyped, leading to incorrect calculations. Converting Dates for Comparison Because the datetime data type contains both time and date components, searching for data rows matching a specific date only, excluding the time component, can sometimes be a bit tricky—especially when you consider that SQL Server stores time values only down to 3/1,000 second. For example, if you want to find all rows where the date is for a certain day, you have to perform a range search for all times within that day. Because a date without a time specified defaults to a time of midnight (00:00:00.000) for that date, the following query doesn’t return all matching rows if any of the data values contain a time other than midnight: `select title_id, pubdate from dbo.titles where pubdate = '2006-01-14'` To be sure to include all rows for a particular date, regardless of the time component stored, you could run a query similar to the following: `select title_id, pubdate from dbo.titles where pubdate between '2006-01-14 00:00:00.0' and '2006-01-14 23:59:59.997'gotitle_id pubdate-------- -----------------------FI3599 2006-01-14 00:00:00.000 ` Now you might be wondering, why use a time of "2006-01-14 23:59:59.997" as the last time of the day? You do so because SQL Server stores datetime values only down to 3/1,000 second. If you enter a time of "2006-01-14 23:59:59.999", SQL Server rounds it up to "2006-01-15 00:00:00.000", and it actually matches any rows with that datetime value, as in this example: `select title_id, pubdate from dbo.titles where pubdate between '2006-01-14 00:00:00.0' and '2006-01-14 23:59:59.999'title_id pubdate-------- -----------------------FI3599 2006-01-14 00:00:00.000FI5162 2006-01-15 00:00:00.000 ` This is one reason you have to be careful when performing date searches. Now you might be wondering why not just use the DATEDIFF function as in the following example: `select title_id, pubdate from dbo.titles where datediff(day, pubdate, '2006-01-14') = 0gotitle_id pubdate-------- -----------------------FI3599 2006-01-14 00:00:00.000` Although this query returns the correct result, the use of the function on the pubdate column may prevent SQL Server from using any indexes that exist on the pubdate column to optimize the query, and it is likely to end up performing a table scan.  To help ensure that your queries are optimized effectively, you need to try to avoid using any functions or expressions on the column in the search argument, and you need to search against constant expressions. Another way to write the preceding query would be to use the date calculations discussed previously in this section. For example, you could use the calculation to determine midnight of the desired date and use that as the inclusive lower bound, and you could use the calculation of midnight of the next day as the noninclusive upper bound and write a query similar to the following: `declare @date datetimeset @date = '2006-01-14'select title_id, pubdate from dbo.titleswhere pubdate >= DATEADD(dd, DATEDIFF(dd,'',@date), '') and pubdate < DATEADD(dd, DATEDIFF(dd,'',@date) + 1, '')gotitle_id pubdate-------- -----------------------FI3599 2006-01-14 00:00:00.000` SQL Server 2008 introduces the date and time data types, as well as the datetime2 data type. The long-awaited date and time data types store just a date value or time value, respectively, making date-only or time-only comparisons much simpler. For example, the previous solution for finding all books published on a specific day can be simplified a bit using the date data type because there is no need to consider a time component: `declare @date dateset @date = '2006-01-14'select title_id, pubdate from dbo.titleswhere pubdate >= @date and pubdate < DATEADD(dd, 1, @date)go` If the pubdate column were defined using the date data type instead of datetime (reasonable because the time of publication of a book is irrelevant), the comparison becomes even simpler: `alter table titles drop constraint DF__titles__pubdate__103673A0drop statistics titles.pubdatealter table titles alter column pubdate date nullalter table titles add constraint DF__titles__pubdate__103673A0 default getdate() for pubdategodeclare @date dateset @date = '2006-01-14'select title_id, pubdate from dbo.titleswhere pubdate = @dategotitle_id pubdate-------- -----------------------FI3599 2006-01-14 00:00:00.000` The datetime2 data type stores the time value down to microseconds and avoids the 3/1,000 second rounding issue that was present with the datetime data type. For example, if you redefine the pubdate column using the datetime2 data type, you avoid the rounding issue and get a single row as expected by the following query: `alter table titles drop constraint DF__titles__pubdate__103673A0alter table titles alter column pubdate datetime2 nullalter table titles add constraint DF__titles__pubdate__103673A0 default sysdatetime() for pubdategoselect title_id, pubdate from dbo.titles where pubdate between '2006-01-14 00:00:00.0' and '2006-01-14 23:59:59.999999'gotitle_id pubdate-------- -----------------------FI3599 2006-01-14 00:00:00.000 `Sorting Results with the GROUPING Function When working with the CUBE or ROLLUP operator, SQL Server generates NULL values for the columns that are being rolled up to generate the aggregate values. When you are viewing the results, however, it can be difficult to determine whether the NULL value shown for a nonaggregate column is the result of a rollup or because the column itself contains a NULL value. Fortunately, SQL Server provides the GROUPING function, which you can use to distinguish between real NULLNULL values that represent a rollup of all values for a column in the result set. values and The GROUPING function returns 1 when the value is grouped and 0 when the column contains a NULL value. In Listing 1, the GROUPING function is used to replace NULL values for the rolled-up columns with ALL. Listing 1. Using the GROUPING Function `SELECT CASE when GROUPING(type) = 1 then 'ALL' else isnull(type, 'Other') END AS type, cast(CASE when (grouping(advance) = 1) then 'ALL' else isnull(convert(varchar(10), advance), 'Unknown') END as varchar(10)) as advance, count(*) AS number FROM DBO.titles where type like '%cook%' or type like 'p%' GROUP BY type, advance WITH rollupgotype advance number------------ ---------- -----------mod_cook 0.00 1mod_cook 15000.00 1mod_cook ALL 2popular_comp Unknown 1popular_comp 7000.00 1popular_comp 8000.00 1popular_comp ALL 3psychology 2000.00 1psychology 2275.00 1psychology 4000.00 1psychology 6000.00 1psychology 7000.00 1psychology ALL 5trad_cook 4000.00 1trad_cook 7000.00 1trad_cook 8000.00 1trad_cook ALL 3ALL ALL 13 ` You can also use the GROUPING function to order the result sets to move all the rollups toward the bottom, as shown in Listing 2. Listing 2. Using the GROUPING Function to Order the Result Sets `SELECT CASE when GROUPING(type) = 1 then 'ALL' else isnull(type, 'Other') END AS type, cast(CASE when (grouping(advance) = 1) then 'ALL' else isnull(convert(varchar(10), advance), 'Unknown') END as varchar(10)) as advance, count(*) AS number FROM DBO.titles where type like '%cook%' or type like 'p%' GROUP BY type, advance WITH rollup ORDER by GROUPING(type), GROUPING(advance)gotype advance number------------ ---------- -----------popular_comp Unknown 1popular_comp 7000.00 1popular_comp 8000.00 1psychology 2000.00 1psychology 2275.00 1psychology 4000.00 1psychology 6000.00 1psychology 7000.00 1trad_cook 4000.00 1trad_cook 7000.00 1trad_cook 8000.00 1mod_cook 0.00 1mod_cook 15000.00 1mod_cook ALL 2trad_cook ALL 3psychology ALL 5popular_comp ALL 3ALL ALL 13 `
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