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With N Kollar , the subquery returns no values and is therefore NULL. The outer query obviously doesn t nd any orders for which empid = NULL and therefore returns an empty set. Note that the query doesn t break (fail) it s a valid query. With N D% , the subquery returns two values (1, 9), and because the outer query expects a scalar, it breaks at run time and generates the following error :

Be suspicious of any expression containing a string that involves an equal sign. String operations in C are nearly always done with strcmp(), strcpy(), strlen(), and related routines. Equal signs often imply some kind of pointer error. In C, assignments do not copy string literals to a string variable. Suppose you have a statement like

The Network folder is your gateway to all available network resources, just as Computer is the gateway to resources stored on your own system. The Network folder contains an icon for each computer on your network; double-click a computer icon to see that computer s shared resources, if any.

The other glaring problem with the previous approach is the amount of redundant code needed to populate the database with test data. Although you could use a SQL script to set up the test data, you would still be faced with the task of testing the state of the database after the test. In the previous approach we tested the database results indirectly via the Hibernate-mapped POJOs. Another approach would be to load some test data, perform tests on that data, check the state of the database before and after, and then clean up. The DbUnit project, created by Manuel Laflamme, can be found at www.dbunit.org. DbUnit provides the functionality to load test data and verify the state of the database after a test. DbUnit uses XML files to provide test data. To load the data, you must initially format the data into DbUnit s specific XML dataset format. The easiest way is to export the data from an existing database in the desired state. DbUnit includes an export utility and Ant task for performing this activity. Listing 8-19 shows an Ant macrodef that uses the DbUnit export subtask to export the data in a database to a file named dataExport.xml in the setup/data/dbunit directory (the name and the location can be overridden). This macrodef is to be added to the macros.xml file created in 3 and provides a good starting point to create data sets for your DbUnit tests. The generated XML file can be renamed and edited for a specific state. I like to use names that tie the file to a specific test like beforeAddressTest.xml and afterAddressTest.xml, for example. The macrodef takes as parameters the JDBC information and the destination and name of the DbUnit XML file to be generated. Listing 8-19. DbUnit Ant Macrodef for Data Export <path id="dbunit.class.path"> <fileset dir="lib/dbunit"> <include name="*.jar"/> </fileset> </path> <!-- DBUnit - dbunit.sourceforge.net --> <taskdef name="dbunit" classname="org.dbunit.ant.DbUnitTask" classpathref="dbunit.class.path" /> <!-- =================================================================== --> <!-- DB Unit Data Export --> <!-- =================================================================== -->

// My sample server just changes all the characters to uppercase // But, you can replace this code with any compute-bound operation data = Encoding.UTF8.GetBytes( Encoding.UTF8.GetString(data, 0, bytesRead).ToUpper().ToCharArray()); // Asynchronously send the response back to the client pipe.BeginWrite(data, 0, data.Length, ae.End(), null); yield return 1; // The response was sent to the client, close our side of the connection pipe.EndWrite(ae.DequeueAsyncResult()); } // Close the pipe }

The product_category table is the linking table that allows implementing the Many-to-Many relationship between product and category. It has two fields, product_id and category_id, which form the primary key of the table. Create and populate the table following the steps in the exercise.

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Throughout the chapter, I will use the Performance database and its tables in my examples. Run the code in Listing 3-1 to create the database and its tables, and populate them with sample data. Note that it will take a few minutes for the code to finish.

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