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SQL> @success.sql begin No error here! Still no error here! declare * ERROR at line 1: ORA-20000: ORA-06502: PL/SQL: numeric or value error: character to number conversion error on assigning a value to n_number in success.sql ORA-06512: at line 23 So, PL/SQL is reporting an ERROR at line 1: That doesn t make much sense. Yet, further down it reports an error at line 23. Hey, that doesn t help much either. It s just a line number by the exception, but it can t be the error it s a blank line! Where did the error occur It took place somewhere in the block of PL/SQL code in scope for the exception-handling section. Regardless, if we rely on our success messages, we can see that the error took place some time after the message Still no error here! and before After the error, and thus narrow down our search to lines 19 through 21. That helps. In addition to using success messages to narrow the scope of an error, I like to keep track of the number of inserts, updates, deletes, and selects and display them at the end of the procedure to verify that something has actually happened. If, on the other hand, you re writing table package or TYPE service methods, you may use put_line() messages during the development process, and then manually comment them out before going into production. Why just comment them out and not delete them altogether By leaving them commented out, you give support programmers a clue about a hard-to-understand or an error-prone section of code, and give them an opportunity to comment the put_line() back into play to help them debug your code in the future. So how can we use failure messages
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In practice, I rely on method raise_application_error() to send a meaningful error number and message to the presentation layer. I simply number application error messages from -20000 to -20199, from top down in my package body. The messages include the variable SQLERRM, a very brief note about what I was doing in the section of code, and the package and/or function or procedure name as a location of the error. The description in the message needs to be brief, because raise_application_error() allows a maximum message size of only 2,048 bytes (255 bytes in earlier versions). Since my message size is limited by raise_application_error(), I supplement the error message with extra lines of output using put_line() before the call to raise_application_error(). With the extra lines of output, I list the values of variables. By including the extra lines of error information, I can call the PL/SQL program unit that raised an error message with the same values as the presentation layer in order to re-create and then correct my code defect. Now that you have some idea how to go about using put_line(), let s see you put it to use.
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It s Your Turn to Use put_line()
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It s your turn to put your knowledge of put_line() and raise_application_error() to work by following these steps: 1. Write an anonymous PL/SQL procedure that adds a number in a number variable to a number in a varchar2 variable together. Here s the hitch: you should spell out the second number in the varchar2 variable so it will raise an exception when added. 2. Block the line of code where you do your addition and then catch the error, list the contents of the variables with pl(), and then raise an application error. 3. Save your script as failure.sql. 4. Execute your script: @failure.sql. Did your script and results turn out similar to mine Listing 7-2 is my solution to this exercise.
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