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

Be aware that using an alias is an all-or-nothing proposition. Once you decide to alias a table, you must
use that alias in every part of the query. This is on a table-by-table basis, but try running some mixed
code and you’ll see what I mean:
SELECT pbe.*, HumanResources.Employee.BusinessEntityID
FROM Person.BusinessEntity pbe
INNER JOIN HumanResources.Employee hre
ON pbe.BusinessEntityID = hre.BusinessEntityID
This seems like it should run fine, but it will give you an error:
Msg 4104, Level 16, State 1, Line 1
The multi-part identifier “HumanResources.Employee.BusinessEntityID” could not be
bound.
Again, you can mix and match which tables you choose to use aliasing on and which you don’t, but once
you make a decision for a given table, you have to be consistent in how you reference that table.
Think back to those bullet points we saw a few pages earlier; the columns from the first table listed in
the JOIN were the first columns returned. Take a break for a moment and think about why that is, and
what you might be able to do to control it.
SQL Server always uses a column order that is the best guess it can make at how you want the columns
returned. In our first query we used one global * operator, so SQL Server didn’t have much to go on. In
that case, it goes on the small amount that it does have — the order of the columns as they exist physically
in the table and the order of tables that you specified in your query. The nice thing is that it is extremely
easy to reorder the columns — we just have to be explicit about it. The simplest way to reorder the columns
would be to change which table is mentioned first, but we can actually mix and match your column order
by simply explicitly stating the columns that we want (even if it is every column), and the order in which
we want them.
Try It Out A Simple JOIN
Let’s try a small query to demonstrate the point:
SELECT pbe.BusinessEntityID, hre.JobTitle, pp.FirstName, pp.LastName
FROM Person.BusinessEntity pbe
INNER JOIN HumanResources.Employee hre
ON pbe.BusinessEntityID = hre.BusinessEntityID
INNER JOIN Person.Person pp
ON pbe.BusinessEntityID = pp.BusinessEntityID
WHERE hre.BusinessEntityID < 4
This yields a pretty simple result set:
BusinessEntityID JobTitle FirstName LastName
----------------- ------------------------------------------------------------
1 Chief Executive Officer Ken Sánchez
2 Vice President of Engineering Terri Duffy
3 Engineering Manager Roberto Tamburello
(3 row(s) affected)
Chapter 4: JOINs
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