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For example, to convert the value January 1, 1980 in variable d_date to a varchar2 data type for variable v_date in the form YYYYMMDD, you could use the following code: v_date := to_char(d_date, 'YYYYMMDD'); It s unlikely that converting from a date (or number) to a varchar2 is ever going to cause an error, but, on the other hand, it s very likely an error will occur if you convert from a varchar2 to a date, as in the following: ... begin d_date := to_date(v_date, 'YYYYMMDD'); exception when OTHERS then pl(SQLERRM); pl('Converting "'||v_date||'" to a date using format YYYYMMDD'); raise_application_error(-20001, SQLERRM|| ' converting v_date to d_date'|| ' in my_program_unit.method'); end; ... In this example, if the character representation of the date in variable v_date is not in the format YYYYMMDD, the to_date() function will raise an appropriate exception. The enclosing PL/SQL block will echo the details to the screen using put_line() via pl(), and then raise an application error that will report exactly where the error occurred in the program. This, in turn, will give you details as to why there was an error, so you are armed with good error information when you start troubleshooting, not some time after you are well into troubleshooting. Just as to_date() and to_char() have formats they can use to specify the conversion parameters, so does to_number(). You can find the details on these formats in the freely available PL/SQL User s Guide and Reference. You can find this and the rest of the Oracle documentation set online at http://otn.oracle.com. So the proper use of the combination of anchors, data type prefixes, and explicit conversion functions will prevent many unexpected errors. Next, let s see how you can better prepare for those that will still eventually arrive.
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Preparing for trouble ahead of time means preparing for troubleshooting ahead of time. How do you prepare for troubleshooting You can do that by using blocks and bread crumbs. You can block (wrap code in its own PL/SQL block) risky code ahead of time. When I say risky code, I mean: Explicit data type conversions Movement of larger character strings to shorter character variables Singleton SQL statements
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And what about bread crumbs You know the fairy tale about Hansel and Gretel, right They dropped bread crumbs on the ground as they walked through the forest, so they could find their way back. Well, I want you to drop code on your listings as you type through your program, so you, too, can find your way back back to the source of a problem. Let s start our discussion of blocking and bread crumbs with the former.
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I just showed you an example of blocking wrapping a small piece of your PL/SQL in its own PL/SQL block in order to catch a raised exception and dealing with it when I discussed explicit conversions. Now I ll show you another example, where I move a larger character string into a smaller character variable. It s not uncommon when coding a data migration or data processing program to move a larger varchar2 variable into a smaller one. In such a situation, you can substring the larger variable in order to blindly truncate the larger string, or you can block the assignment so it raises an exception that you can deal with intelligently. Here s an example of the latter: declare v_large v_small varchar2(80); varchar2(30);
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begin -- I'm assigning the variable in the executable section, because -- assignment errors in the declaration section are also very hard -- to troubleshoot! -- 12345678901234567890123456789012345678901234567890123456789012345678 v_large := 'This is a large string of characters, at least longer than 30 bytes!'; -- Now let's raise an exception begin -- This won't work! 68 bytes won't fit in 30 bytes! v_small := v_large; exception when OTHERS then pl(SQLERRM); pl('Moving v_large, length '|| to_char(length(v_large))||' into v_small.'); raise_application_error(-20001, SQLERRM|| ' on v_small := v_large'|| ' in my anonymous procedure'); end; pl(v_small); end; /
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