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

Chapter 3: The Foundation Statements of T-SQL
Before we leave the COUNT function, we had better see it in action with the GROUP BY clause.
For this next example, you’ll need to load and execute the BuildAndPopulateEmployee2
.sql file included with the downloadable source code (you can get that from either
the wrox.com or professionalsql.com websites).
All references to Employees in the following examples should be aimed at the new
Employees2 table rather than Employees.
Let’s say our boss has asked us to find out the number of employees that report to each manager. The
statements that we’ve done thus far would either count up all the rows in the table (COUNT(*)) or all the
rows in the table that didn’t have null values (COUNT(ColumnName)). When we add a GROUP BY clause,
these aggregators perform exactly as they did before, except that they return a count for each grouping
rather than the full table. We can use this to get our number of reports:
SELECT ManagerID, COUNT(*)
FROM HumanResources.Employee2
GROUP BY ManagerID;
Notice that we are grouping only by the ManagerID — the COUNT() function is an aggregator and,
therefore, does not have to be included in the GROUP BY clause.
ManagerID
----------- -----------
NULL 1
1 3
4 3
5 4
(4 row(s) affected)
Our results tell us that the manager with 1 as his/her ManagerID has 3 people reporting to him or her,
and that 3 people report to the manager with ManagerID 4 as well as 4 people reporting to ManagerID 5.
We are also able to tell that one Employee record had a NULL value in the ManagerID field. This employee
apparently doesn’t report to anyone (hmmm, president of the company I suspect?).
It’s probably worth noting that we, technically speaking, could use a GROUP BY clause without any kind
of aggregator, but this wouldn’t make sense. Why not? Well, SQL Server is going to wind up doing work
on all the rows in order to group them, but functionally speaking you would get the same result with a
DISTINCT option (which we’ll look at shortly), and it would operate much faster.
Now that we’ve seen how to operate with groups, let’s move on to one of the concepts that a lot of people
have problems with. Of course, after reading the next section, you’ll think it’s a snap.
Placing Conditions on Groups with the HAVING Clause
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Up to now, all of our conditions have been against specific rows. If a given column in a row doesn’t have
a specific value or isn’t within a range of values, then the entire row is left out. All of this happens before
the groupings are really even thought about.