Learning the Basics of PL/SQL - Page 3

Alternatives to Retyping

Until now, you have been retyping each PL/SQL block as you tried it. If you made a mistake, you had to type the code all over again. There are some alternatives to typing PL/SQL straight into SQL*Plus. Depending on your personal preferences, and on what you are trying to do, there are three basic ways to go about this:

• Cut and paste from Notepad.

• Execute a text file using the SQL*Plus @ command.

• Use the SQL*Plus EDIT command.

The first method involves running Windows Notepad, typing your PL/SQL code (or SQL queries) into it, and then copying and pasting from Notepad into SQL*Plus to execute the desired code. This method is ideal for experimenting with short snippets of PL/SQL code and SQL queries. You can keep several related items in the same text file where you can easily call them up when you want to work on them.

The second method makes use of a SQL*Plus command to execute a file. For example, if you have a text file named test.sql with the code from Listing 1.2, you could execute that file by typing this command:

SQL> @c:atest

The resulting output would look like:

The variable X =
65400

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Note - When you're executing a file, the default file extension is .SQL. SQL*Plus looks for the file first in the default directory and then follows a search path that you can define. How you define this path is operating system--specific and outside the scope of this book. For details, you should consult the SQL*Plus User's Guide and also your operating system documentation.

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Executing commands from a file like this is most useful in cases where you are re-creating a stored procedure, function, or database trigger and you have the definition already stored in its own text file.

The third option involves using the SQL*Plus EDIT command to invoke your system's text editor. Under Windows, this will be Notepad unless you have specifically defined a different editor. When you issue the EDIT command, SQL*Plus will launch Notepad and automatically place in it the text of the most recently executed PL/SQL block or SQL statement. See Figure 1.3 for an example of this.

Using the SQL*Plus EDIT command.

After you've brought up Notepad, you can edit the PL/SQL block to your satisfaction and then exit from Notepad, being sure to save the file. When you save your file, SQL*Plus will not immediately reexecute it. It is merely placed in an internal buffer. You must use the / command, by typing / on a line by itself, in order to execute the code you just edited.

Using the EDIT command works well as long as you keep in mind one important thing. SQL*Plus remembers only the most recent SQL statement or PL/SQL block. If you have been working on a PL/SQL block, and you execute just one SQL statement, that statement will replace the PL/SQL block you have been editing.

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Caution - Do not allow the SQL*Plus buffer to contain your only copy of a long procedure. It's too easy to enter a SQL command without thinking and wipe out the much longer PL/SQL procedure you have been developing.

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Which of these three methods you choose is up to you, and depends in part on your personal preferences. You are likely to find the first method, copying and pasting between Notepad and SQL*Plus, most useful during the first few chapters of this book. As you write larger PL/SQL functions and procedures, you will find yourself gravitating toward keeping each in its own file.

Writing Your First Function

Perhaps one of the most useful things you can do with your knowledge of PL/SQL is to use it to write stored functions and stored procedures. Encapsulating the code you wrote earlier into a stored function enables you to compile it once and store it in the database for future use. The next time you want to run that PL/SQL block, all you need to do is invoke the function. Using SQL*Plus, type in the input code shown in Listing 1.3, which will create a PL/SQL function to return the value that was output by Listing 1.2.

Listing 1.3 The SS_THRESH Function

1: CREATE OR REPLACE FUNCTION ss_thresh
2: RETURN NUMBER AS
3:  x   NUMBER;
4: BEGIN
5:  x := 72600;
6:  RETURN x;
7: END;
8: /

Function created

Compare the code in Listing 1.3 to that in Listing 1.2. Notice that the keyword DECLARE has been replaced in lines 1 and 2 by the words CREATE OR REPLACE FUNCTION ss_thresh RETURN NUMBER AS. This will be explained further in Day 3. Also notice that the calls to dbms_output.put_line() have been replaced by the RETURN command (line 6), which returns the value of the variable X to the caller. The only output from Listing 1.3 is a confirmation that the function has been successfully created, which is shown in line 9.

Notice that Oracle has created the function. SQL*Plus indicates this by displaying the words Function created.

Finding Compilation Errors

You probably were able to type in the code from Listing 1.3 and create the SS_THRESH function with no errors. However, that might not have been the case. To show you how to deal with an error, Listing 1.4 contains the same code as Listing 1.3, but with one small error.

Listing 1.4 The SS_THRESH Function with an Error

 1: CREATE OR REPLACE FUNCTION ss_thresh
 2: RETURN NUMBER AS
 3:  x   NUMBER;
 4: BEGIN
 5:  x = 72600;
 6:  RETURN x;
 7: END;
 8: /

Warning: Function created with compilation errors.

Unlike most compilers, which will display a listing of errors found in source code, Oracle stores any errors it finds in a database table named USER_ERRORS. If you want to see the specific details, and you may well, you need to retrieve the error listing yourself. Use the SQL*Plus command SHOW ERRORS, as shown in Listing 1.5, to do this.

Listing 1.5 The SHOW ERRORS Command

 1: SHOW ERRORS
Errors for FUNCTION SS_THRESH:

 LINE/COL ERROR
 -------- ---------------------------------------------------------------
 5/5      PLS-00103: Encountered the symbol "=" when expecting one of the
          following:
          := . ( @ % ;
          The symbol ":= was inserted before "=" to continue.

As you can see, the error listing has two columns of output. The first column contains the line number where the error occurred and also the character position within that line. The second column contains the specific error message. In this example, the error occurred in line 5 at the fifth character position. The error message tells you that Oracle encountered an equal sign when it was really expecting something else. That "something else," in this case, is the assignment operator, represented by :=.

Figure 1.4 shows the SQL*Plus screen as it would look after executing Listings 1.4 and 1.5.

Error listing for SS_THRESH.

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Tip - Typing = instead of := is a common mistake to make, especially if you also program in other languages that do use = for assignment.

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Displaying the Function's Return Value

Now that you have written and compiled the function, it's time to execute it and see the results. The easiest way to do this using SQL*Plus is to issue the following SQL command:

SELECT SS_THRESH FROM DUAL;
SS_THRESH
---------
  72600

The SS_THRESH function does not have any parameters, so be sure not to add any parentheses when you call it. In other words, don't use SS_THRESH() because Oracle will return an error. The table DUAL is a special Oracle table that always exists, always has exactly one row, and always has exactly one column. It's the perfect table to use when experimenting with functions. Selecting the function from the DUAL table causes the function result to be displayed.

Can Even This Simple Function Be Useful?

The SS_THRESH function is a very simple function, and you might rightly wonder if something so absurdly simple can be useful. The value this function returns is the Social Security Contribution and Benefit Base, a value that changes from year to year. If you were a programmer working on a payroll system and needed to write several queries using this value, you could use a function like this to encapsulate this information. To encapsulate information means to embed it within a function so that values like this don't need to be replicated all through your code, and so that any changes can be made in one central place. There's another benefit to this approach. Your queries become more self-documenting. It's a bit easier to remember six months later what you meant when you see

SELECT * FROM employee_table
 WHERE emp_salary > SS_THRESH;

than if you had simply hard-coded the value

SELECT * FROM employee_table
 WHERE emp_salary > 72600;