How to Visualize What is Possible and Impossible Within an SQL Statement in Java

Produce QR in Java How to Visualize What is Possible and Impossible Within an SQL Statement
How to Visualize What is Possible and Impossible Within an SQL Statement
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Take a look at the following error message:
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Column e.Firstname is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.
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How many times have you seen something like this You go back and edit your SQL and give it another try, but it either fails again or doesn t give you what you re looking for. So you give it another try. . . .
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9 Putting Databases to Work
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Sound familiar If you follow what we say after you finish reading this chapter, you never encounter this problem again.
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SQL s processing order of precedence
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To understand exactly what you can and cannot do with SQL, you must first understand the order in which SQL processes the clauses of an SQL statement. You can use Listing 9-6 as an example of the order that SQL follows.
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Listing 9-6: An example query for discovering how SQL processes statements
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SELECT c.CompanyName, e.LastName + , + e.FirstName AS EmployeeName, e.Salary, count(*) as NumDependants FROM Company c INNER JOIN Employee e ON c.CompanyID = e.CompanyID INNER JOIN Dependant d ON e.SSN = d.SSN WHERE e.DateOfBirth < 01/01/82 GROUP BY c.CompanyName, e.LastName, e.FirstName, e.Salary HAVING Count(*) >= 2 ORDER BY EmployeeName ASC
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The following sections show you how SQL processes Listing 9-6, step-by-step.
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Step 1: The FROM clause
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Before SQL can do anything with data, it must get that data, so the FROM clause is the first to be processed in the statement. The intermediate work product of the FROM clause of Listing 9-6 is shown in Figure 9-7. Remember that the result of the FROM clause is a single set of data in the form of a simple table, regardless of how many tables are joined together in the FROM clause. Refer to the section Understanding All Relational Result Sets, earlier in this chapter, for the details of how JOINs are processed in the FROM clause.
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Step 2: The WHERE clause
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The intermediate work product of the FROM clause is then fed to the WHERE clause, which further processes this set of data.
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Part II Using ColdFusion MX with Databases
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Figure 9-7: The intermediate work product of the FROM clause of Listing 9-6. The purpose of the WHERE clause is to select rows that satisfy a specific criterion or criteria. Only those rows in the intermediate work product of the FROM clause that satisfy the WHERE clause criteria can pass to the next clause in SQL s processing order of precedence. The intermediate work product of the WHERE clause of Listing 9-6 is shown in Figure 9-8.
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Figure 9-8: The intermediate work product of the WHERE clause of Listing 9-6. If an SQL statement doesn t contain a WHERE clause, the intermediate work product of the FROM clause instead passes to the next clause in the statement.
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Step 3: The GROUP BY clause
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The intermediate work product then passes to the GROUP BY clause, if one is present in the statement. The purpose of the GROUP BY clause is to group together rows in the intermediate work product based on homogenous values in the columns that define the group.
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9 Putting Databases to Work
In other words, if a statement groups by the CompanyName column, all rows containing the same value as CompanyName belong to the same group after the GROUP BY clause finishes processing. Similarly, if a statement groups by the LastName and FirstName columns, all rows containing the same combination of LastName and FirstName values belong to the same group after the GROUP BY clause is finished processing. But what do we mean by groups If the result of every clause of an SQL statement is a simple table, where do these groups fit in the table that results from a GROUP BY clause Actually, each group becomes a single row of the intermediate work product of the GROUP BY clause, and each group s constituent rows are like separate little invisible tables turned sideways and sitting behind each group, as shown in Figure 9-9. This invisible third dimension to the intermediate work product of the GROUP BY clause is available to the HAVING clause and aggregate functions, as you learn in the section Step 4: The HAVING clause, later in this chapter. The invisible third dimension is an internal working structure for these purposes only and is never returned as part of a final query result set. So the intermediate work product of the GROUP BY clause of Listing 9-6 is as shown in Figure 9-10.