Debugging a PL/SQL Procedure

If you want to practice debugging a PL/SQL procedure with SQL Developer, create a procedure that is like the list_a_rating procedure that you created in Creating a PL/SQL Procedure, but with a logic error. (The coding is also deliberately inefficient, to allow the display of the rating in a variable.)

To create this procedure, if you are not already connected, connect to the database as the user for the schema you are using for this tutorial. Right-click the Procedures node in the schema hierarchy on the left side, select Create New PL/SQL Procedure, and enter the following information using the Create PL/SQL Procedure dialog box.

Object Name: list_a_rating2

Click OK. A Source window for the new procedure is opened. Enter (or copy and paste) the following procedure text:

PROCEDURE  list_a_rating2(in_rating IN NUMBER) AS
  matching_title VARCHAR2(50);
  matching_rating NUMBER;
  TYPE my_cursor IS REF CURSOR;
  the_cursor my_cursor;
  rating_cursor my_cursor;
BEGIN
  OPEN the_cursor FOR 'SELECT title FROM books WHERE 
    rating <= :in_rating' USING in_rating;
  OPEN rating_cursor FOR 'SELECT rating FROM books WHERE 
    rating <= :in_rating' USING in_rating;
  DBMS_OUTPUT.PUT_LINE('All books with a rating of ' || in_rating || ':');
  LOOP
    FETCH the_cursor INTO matching_title;
    FETCH rating_cursor INTO matching_rating;
    EXIT WHEN the_cursor%NOTFOUND;
    DBMS_OUTPUT.PUT_LINE(matching_title);
  END LOOP;
  CLOSE the_cursor;
  CLOSE rating_cursor;
END;

This procedure contains a logic error in the definition of the_cursor: it selects titles where the rating is less than or equal to a specified rating, whereas it should select titles only where the rating is equal to the specified rating.

Click the Save icon to save the procedure.

Assume that you wanted to run this procedure and list all books with a rating of 10. Right-click LIST_A_RATING2 in the Connections navigator hierarchy display and select Run. In the Run PL/SQL dialog box, change IN_RATING => IN_RATING to IN_RATING => 10, and click OK. In the Log window, however, you see unexpected output: many titles are listed, including some with ratings other than 10. So, you decide to debug the procedure.

To debug the procedure, follow these steps:

Click LIST_A_RATING2 in the Connections navigator, then right-click and select Compile for Debug.
In the code tab, click the Edit icon (pencil and paper graphic).
Set two breakpoints by clicking in the left margin (left of the thin vertical line) beside each of these two lines: FETCH the_cursor INTO matching_title; and FETCH rating_cursor INTO matching_rating;. (Clicking in the left margin toggles the setting and unsetting of breakpoints.) This will enable you to see the values of the matching_title and matching_rating variables as execution proceeds in debug mode.
Right-click LIST_A_RATING2 in the Connections navigator hierarchy display and select Debug. Ensure that the line IN_RATING => IN_RATING has been changed to IN_RATING => 10, and click OK.
In the debugging toolbar, click the Resume icon to have execution proceed, stopping at the next breakpoint; note the display in the Smart Data tab in the lower right part of the window. Repeatedly click the Resume icon, noticing especially the value of the rating variable as each row is processed. You will notice the first incorrect result when you see that the title Get Rich Really Fast is included, even though its rating is only 1 (obviously less than 10). (See the screen illustration with debugging information in Running and Debugging Functions and Procedures.)
When you have enough information to fix the problem, you can click the Terminate icon in the debugging toolbar.

From this debugging session, you know that to fix the logic error, you should change rating <= :in_rating to rating = :in_rating in the definition of the_cursor.