More Advanced Database Techniques in Java

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Notice that you don t specify a datasource if dbtype= Query . Instead, you just refer to the names of the queries as if they were table names, and ColdFusion Server knows to access its own queries rather than look in a specific datasource for tables.
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As useful as QoQ is in certain circumstances, you need to consider a few caveats and restrictions in using it, as the following list describes: OUTER JOINs are not supported. SQL-92 JOIN syntax (for example, Table1 INNER JOIN Table2 ON Key1 = Key2) is not supported Table-name aliases are not supported, which means that you must always fully qualify table names in relational JOINs. If an aggregate function is retrieved as one of the columns in your result set, that function cannot be used in GROUP BY or HAVING clauses. Because QoQ text matches are case-sensitive, you must compare a functionally converted value of a column against the matching criterion to ensure positive matches (for example, WHERE Upper(LastName) LIKE SMI% ). ODBC-formatted dates and times are not supported. QoQ also has a couple enhancements over what was available in ColdFusion 5, as the following list describes: If an aggregate function is aliased, the alias must be used in the GROUP BY and HAVING clauses rather than the function itself. This behavior is counter to logical SQL statement processing, as the SELECT clause where aliases are defined is internally processed after the GROUP BY and HAVING clauses, so don t try to use the same flawed syntax with your normal database queries. The LIKE conditional works more like a normal database LIKE conditional, which supports search patterns containing wildcard characters. So a typical relational join with QoQ performs an INNER JOIN in the WHERE clause and uses long-form table naming, as the following example shows:
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SELECT Customer.CompanyName AS CompanyName, SalesOrder.SalesOrderID AS SalesOrderID, SalesOrder.SaleDate AS SaleDate, SalesOrder.OrderTotal AS OrderTotal FROM Customer, SalesOrder WHERE Customer.CustomerNumber = SalesOrder.CustomerNumber
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You may also notice that we aliased the column names in this example to be the same as their original names. If we didn t do so in ColdFusion 5, an error would be thrown in trying to resolve the variable name. If instead of providing an alias, we fully resolved the column as TableName.ColumName in the CFOUTPUT, ColdFusion 5 would incorrectly interpreted this syntax as QueryName.ColumnName and instead would repeat the value in the first row of the table for each row of the output. You prevent all such problems by simply providing an alias for each column name in a query join. This problem with fully resolved column names was fixed with ColdFusion MX, but you should still alias your columns for both ease of programming and backward compatibility. So, now that you know how to query a query . . . don t! At least, not unless you truly need to. Always remember that QoQ is a low-performance feature that s not designed for serious query work. If you re thinking that you can gain performance by making one big database server query and then using QoQ to extract data from the resulting ColdFusion query, think again. Why are you averse to going back to the database server for a fresh query instead of extracting a subset of your existing ColdFusion query Take our advice: Turn on your performance metrics and load-test the system both ways. Do so and you clearly see the difference in performance. Most of the time, going back to the database server for a fresh query wins.
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The same admonition about evaluating and otherwise post-processing ColdFusion query results within CFLOOPs holds just as true for QoQ. You want to return results from your database server in the exact format in which they are displayed, if at all possible. Under most circumstances, if you can program the data to look the way that you want in ColdFusion, you can probably also do the same thing directly in the database by using SQL (or whatever extensions to SQL that your database server gives you).
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