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

This join-based syntax, for example, would have yielded exactly the same results (subject to possible sort
differences). So why, then, would we need this new syntax? Performance — plain and simple.
When you use the EXISTS keyword, SQL Server doesn’t have to perform a full row-by-row join. Instead, it
can look through the records until it finds the first match and stop right there. As soon as there is a single
match, the EXISTS is true, so there is no need to go further.
Let’s take a brief look at things the other way around — that is, what if our query wanted the persons
who were not employees? Under the join method that we looked at in Chapter 4, we would have had
to make some significant changes in the way we went about getting our answers. First, we would have to
use an outer join. Then we would perform a comparison to see whether any of the Employee records
were NULL.
It would look something like this:
SELECT pp.BusinessEntityID, LastName + ', ' + FirstName AS Name
FROM Person.Person pp
LEFT JOIN HumanResources.Employee hre
ON pp.BusinessEntityID = hre.BusinessEntityID
WHERE hre.BusinessEntityID IS NULL;
Which returns 19,682 rows:
BusinessEntity ID Name
-------------- ------------------------------------------------------------------
293 Abel, Catherine
295 Abercrombie, Kim
2170 Abercrombie, Kim
…
…
2088 Zugelder, Judy
12079 Zukowski, Jake
2089 Zwilling, Michael
(19682 row(s) affected)
To do the same change over when we’re using EXISTS, we add only one word to the original EXIST
query — NOT:
SELECT BusinessEntityID, LastName + ', ' + FirstName AS Name
FROM Person.Person pp
WHERE NOT EXISTS
(SELECT BusinessEntityID
FROM HumanResources.Employee hre
WHERE hre.BusinessEntityID = pp.BusinessEntityID);
And we get back those exact same 19,682 rows.
Chapter 7: Adding More to Our Queries
The performance difference here is, in most cases, even more marked than with the inner join. SQL
Server just applies a little reverse logic versus the straight EXISTS statement. In the case of the NOT we’re
now using, SQL can still stop looking as soon as it finds one matching record — the only difference is
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