9-8: An example without a HAVING clause in Java

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Listing 9-8: An example without a HAVING clause
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SELECT e.LastName + , + e.FirstName AS EmployeeName, count(*) as NumDependantsAdded13OrOlder FROM Employee e INNER JOIN Dependant d
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Continued
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Part II Using ColdFusion MX with Databases
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Listing 9-8 (continued)
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ON e.SSN = d.SSN GROUP BY e.LastName, e.FirstName HAVING Min(d.Age) > 13 ORDER BY EmployeeName ASC
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Figure 9-15: The result set from Listing 9-8. Remember that the preceding explanations are conceptualizations, not definitions. We are using terms such as invisible third dimension, constituent rows, and sitting behind very loosely to help you visualize what is happening behind the scenes, because without this visualization, your coding is likely to be a ready-fire-aim affair that doesn t give you what you want.
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When you can and cannot use aggregate functions
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To aggregate means to gather multiple elements into a whole. An aggregate function operates on a set of rows and returns a single value representing the aggregate of those rows. For instance, Avg(Salary) returns the average salary of the selected employee rows, so Avg( ) is considered an aggregate function. Aggregate functions are also known as set functions because they operate over a set of data. The best way to determine when you can and cannot use aggregate functions is to carefully listen to the English language version of the question (that is, the query) that you are asking the database. What is the average salary of all Employees born after July 4, 1975 is, for example, a straightforward, well-formed question, so it easily fits into an SQL statement, as the following code shows:
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SELECT Avg(Salary) FROM Employee WHERE DateOfBirth > 07/04/1975
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But what about What is the name of every Employee and the average salary of all Employees born after July 4, 1975 Is this a valid question Actually, it is two separate questions that are unnaturally smushed together. The following query that would attempt to answer this question is equally unnatural and would, therefore, throw an error:
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9 Putting Databases to Work
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SELECT LastName + , + FirstName AS EmployeeName, Avg(Salary) FROM Employee WHERE DateOfBirth > 07/04/1975
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How can you retrieve both individual employees and the average of all employees as a single query result set You can t! These are two different sets of data with two totally different dimensions, and one has absolutely nothing at all to do with the other. The fact that both questions are about Employees makes no difference. As do all functions, the Avg() function returns a single value in this case, the average of the values in the Salary column for the set of data described by the WHERE clause. That s one value, meaning that it appears exactly one time in the final query result. In other words, it has a dimension of one row by one column. On the other hand, consider the column defined by the following code:
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LastName + , + FirstName AS EmployeeName
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EmployeeName is not a function, nor does it operate over a set of data. As you would expect, this query column definition returns one value for each row in the data described by the WHERE clause. In other words, it has a dimension of N rows by one column. Clearly, the averaging function cannot co-exist with the column display in the same SQL statement because they are of different dimensions. These same rules of operation hold for all aggregate functions, such as Min(), Max(), Sum(), and so on.
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The rules change if GROUP BY is present
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If GROUP BY is present in an SQL statement, the set of data over which a set function operates changes from the one entire data set described by the statement to each separate set of constituent rows sitting behind each group. Do you see why that visualization is so useful By showing that invisible third dimension, you can now visualize the result of the set function for each set or group of rows in the result set. If you want to find the average age at which the Dependants of each Employee were recorded in the database, for example, you would execute the following query:
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SELECT e.LastName + , + e.FirstName AS EmployeeName, Avg(d.Age) as AverageAge FROM Employee e INNER JOIN Dependant d ON e.SSN = d.SSN GROUP BY e.LastName, e.FirstName
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And the result would be as follows:
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