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

Chapter 3: The Foundation Statements of T-SQL
66
As you can see, this cut down the size of your list substantially and made the contents of the list more
relevant. Another side benefit of this query is that it will actually perform better than the first one. Why?
Well, I go into that later in the book when I discuss performance issues further, but for now, suffice it to
say that not having to return every single row means that SQL Server doesn’t have to do quite as much
work in order to meet the needs of this query.
As the old commercials on television go, “But wait! There’s more!” We’re not done with DISTINCT yet.
Indeed, the next example is one that you might be able to use as a party trick to impress your programmer
friends. You see, this is one that an amazing number of SQL programmers don’t even realize you
can do. DISTINCT can be used as more than just a predicate for a SELECT statement. It can also be used
in the expression for an aggregate. What do I mean? Let’s compare three queries.
First, grab a row count for the SalesOrderDetail table in AdventureWorks:
SELECT COUNT(*)
FROM Sales.SalesOrderDetail;
If you haven’t modified the SalesOrderDetail table, this should yield you around 121,317 rows.
Now run the same query using a specific column to COUNT:
SELECT COUNT(SalesOrderID)
FROM Sales.SalesOrderDetail;
Since the SalesOrderID column is part of the key for this table, it can’t contain any NULLs (more on this
in the chapter in indexing). Therefore, the net count for this query is always going to be the same as the
COUNT(*) — in this case, it’s 121,317.
Key is a term used to describe a column or combination of columns that can be used
to identify a row within a table. There are actually several different kinds of keys
(we’ll see much more on these in Chapters 6, 8, and 9), but when the word “key” is
used by itself, it is usually referring to a table’s primary key. A primary key is a column
(or group of columns) that is (are) effectively the unique name for that row.
When you refer to a row using its primary key, you can be certain that you will get
back only one row, because no two rows are allowed to have the same primary key
within the same table.
Now for the fun part. Modify the query again:
SELECT COUNT(DISTINCT SalesOrderID)
FROM Sales.SalesOrderDetail;
Now we get a substantially different result:
-----------
31465
(1 row(s) affected)