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

Chapter 4: JOINs
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Uh, oh — this is a problem. You get an error back:
Msg 209, Level 16, State 1, Line 1
Ambiguous column name ‘BusinessEntityID’.
Why did JobTitle work and BusinessEntityID not work? For just the reason SQL Server has
indicated — our column name is ambiguous. While JobTitle exists only in the Employee table,
BusinessEntityID appears in both tables. SQL Server has no way of knowing which one we want. All
the instances where we have returned BusinessEntityID up to this point have been resolvable: that is,
SQL Server could figure out which column was which. In the first query (where we used a plain * operator),
we asked SQL Server to return everything — that would include both BusinessEntityID columns,
so no name resolution was necessary. In our second example (where we qualified the * to be only for
BusinessEntity), we again said nothing specifically about which BusinessEntityID column to
use — instead, we said pull everything from the Contact table and BusinessEntityID just happened
to be in that list. JobTitle was resolvable because there was only one JobTitle column, so that was
the one we wanted.
When we want to refer to a column where the column name exists more than once in our JOIN result,
we must fully qualify the column name. We can do this in one of two ways:
❑ Provide the name of the table that the desired column is from, followed by a period and the
column name (Table.ColumnName)
❑ Alias the tables, and provide that alias, followed by a period and the column name
(Alias.ColumnName), as shown in the previous example
The task of providing the names is straightforward enough — we’ve already seen how that works with
the qualified * operator, but let’s try our BusinessEntityID query again with a qualified column name:
SELECT Person. BusinessEntity.*, HumanResources.Employee. BusinessEntityID
FROM Person. BusinessEntity
INNER JOIN HumanResources.Employee
ON Person. BusinessEntity. BusinessEntityID
= HumanResources.Employee. BusinessEntityID
Now things are working again and the BusinessEntityID from the Employee table is added to the far
right-hand side of the result set.
Aliasing the table is only slightly trickier, but can cut down on the wordiness and help the readability of
your query. It works almost exactly the same as aliasing a column in the simple SELECTs that we did in
the previous chapter — right after the name of the table, we simply state the alias we want to use to refer
to that table. Note that, just as with column aliasing, we can use the AS keyword (but for some strange
reason, this hasn’t caught on as much in practice):
SELECT pbe.*, hre.BusinessEntityID
FROM Person.BusinessEntity pbe
INNER JOIN HumanResources.Employee hre
ON pbe.BusinessEntityID = hre.BusinessEntityID
Run this code and you’ll see that we receive the exact same results as in the previous query.