Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

Chapter 10: Views
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(51 row(s) affected)
The DATEADD and CAST Functions
The join, while larger than most of the ones we’ve done this far, is still pretty straightforward. We keep
adding tables, joining a column in each new table to a matching column in the tables that we’ve already
named. As always, note that the columns do not have to have the same name — they just have to have
data that relates to one another.
Since this was a relatively complex join, let’s take a look at what we are doing in the query that supports
this view.
The WHERE clause is where things get interesting:
WHERE CAST(soh.OrderDate AS Date) =
CAST(DATEADD(day,-1,GETDATE()) AS Date)
It’s a single comparison, but we used several functions to come up with our result.
It would be very tempting to just compare the OrderDate in the SalesOrderHeader table to GET-
DATE() (today’s date) minus one day — the subtraction operation is what the DATEADD function is
all about. DATEADD can add (you subtract by using negative numbers) any amount of time you want to
deal with. You just tell it what date you want to operate on, and what unit of time you want to add to
it (days, weeks, years, minutes, and so on). On the surface, you should just be able to grab today’s date
with GETDATE() and then use DATEADD to subtract one day. The problem is that GETDATE() returns
the datetime data type and therefore includes the current time of day, so we would get back only rows
from the previous day that happened at the same time of day down to 3.3333 milliseconds (the level of
precision for a datetime field) — not a likely match. So we took things one more step and used the
CAST function to cast the dates on both sides of the equation to a data type that does not support timeof-day-less
before comparison. Therefore, the view will show any sale that happened any time on the
previous date.
Note that this code would not have worked on SQL Server 2005 and earlier installations, as the Date
data type is new with SQL Server 2008. In older versions of SQL Server, we would use the CONVERT
function to change the date to a varchar formatted without the time, and then perform the comparison.
Using a View to Change Data — Before INSTEAD OF Triggers
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As we’ve said before, a view works mostly like a table does from an in-use perspective (obviously, creating
them works quite a bit differently). Now we’re going to come across some differences, however.