PL/SQL Cursor Attributes

PL/SQL is Oracle’s procedural extension to SQL, by which the developer performs complex applications in databases with minimal ease. Some of its powerful features are using curs

ors for the row-by-row processing of results from SQL queries. Under the area of cursors, there exist special attributes from PL/SQL that enhance control over cursor operations. This article on focus to Understanding PL/SQL Cursor Attributes such as %FOUND, %NOTFOUND, and %ROWCOUNT. In this section, we shall discuss how these attributes are used, give examples for each of their usage, and explain why they are so important in PL/SQL programming.

Understanding Cursors in PL/SQL

Before diving into cursor attributes, it’s essential to understand what a cursor is in PL/SQL. A cursor is a database object that allows you to retrieve and manipulate data row by row. Cursors can be categorized into two main types:

1. Implicit Cursors

Implicit cursors are automatically created by PL/SQL when executing a SQL statement that returns a single row or multiple rows. They handle the retrieval process but do not provide the same level of control as explicit cursors.

2. Explicit Cursors

Explicit cursors are defined and controlled by the developer. They allow for more granular manipulation of the data returned by SQL queries. Explicit cursors require the following steps:

• Declaration: Define the cursor.
• Opening: Open the cursor to establish a context for the data retrieval.
• Fetching: Retrieve rows one at a time.
• Closing: Close the cursor once the data has been processed.

Importance of Cursor Attributes

Cursor attributes provide valuable information about the state of the cursor and the results of fetch operations. They are especially useful in controlling the flow of logic in PL/SQL programs. The three primary cursor attributes are:

• %FOUND: Indicates whether the last fetch operation returned a row.
• %NOTFOUND: Indicates whether the last fetch operation did not return a row.
• %ROWCOUNT: Returns the number of rows fetched by the cursor.

Cursor Attributes in Detail

1. The %FOUND Attribute

The %FOUND attribute returns TRUE if the last fetch operation retrieved a row and FALSE if it did not. This attribute is particularly useful for determining if the cursor has any remaining rows to process.

Syntax

cursor_name%FOUND

Example of Using %FOUND

DECLARE
  CURSOR emp_cursor IS
    SELECT employee_id, employee_name FROM employees;
  
  v_emp_id employees.employee_id%TYPE;
  v_emp_name employees.employee_name%TYPE;
BEGIN
  OPEN emp_cursor;
  FETCH emp_cursor INTO v_emp_id, v_emp_name;
  
  IF emp_cursor%FOUND THEN
    DBMS_OUTPUT.PUT_LINE('Employee ID: ' || v_emp_id || ', Name: ' || v_emp_name);
  ELSE
    DBMS_OUTPUT.PUT_LINE('No employee found.');
  END IF;

  CLOSE emp_cursor;
END;

Explanation of the Example

In this example:

• We declare an explicit cursor emp_cursor to select employee details from the employees table.
• After opening the cursor, we fetch the first row.
• We then check whether the fetch operation was successful by the %FOUND attribute.
• Depending on whether a row was found, an appropriate message is printed.

2. The %NOTFOUND Attribute

The %NOTFOUND attribute is the complement of %FOUND. Returns TRUE if the last fetch operation could not return any rows, false if it could.

Syntax

cursor_name%NOTFOUND

Example of Using %NOTFOUND

DECLARE
  CURSOR emp_cursor IS
    SELECT employee_id, employee_name FROM employees;
  
  v_emp_id employees.employee_id%TYPE;
  v_emp_name employees.employee_name%TYPE;
BEGIN
  OPEN emp_cursor;
  
  LOOP
    FETCH emp_cursor INTO v_emp_id, v_emp_name;
    
    IF emp_cursor%NOTFOUND THEN
      DBMS_OUTPUT.PUT_LINE('No more employees to fetch.');
      EXIT;
    END IF;
    
    DBMS_OUTPUT.PUT_LINE('Employee ID: ' || v_emp_id || ', Name: ' || v_emp_name);
  END LOOP;

  CLOSE emp_cursor;
END;

Explanation of the Example

In this example:

• We use a loop to fetch all employees from the cursor.
• After each fetch, we check if the %NOTFOUND attribute is TRUE, indicating that there are no more rows to fetch.
• If no rows are found, a message is printed, and the loop exits.

3. The %ROWCOUNT Attribute

The %ROWCOUNT attribute returns the number of rows fetched from the cursor. This attribute is useful for tracking how many records have been processed during the cursor’s lifetime.

Syntax

cursor_name%ROWCOUNT

Example of Using %ROWCOUNT

DECLARE
  CURSOR emp_cursor IS
    SELECT employee_id, employee_name FROM employees;
  
  v_emp_id employees.employee_id%TYPE;
  v_emp_name employees.employee_name%TYPE;
BEGIN
  OPEN emp_cursor;
  
  LOOP
    FETCH emp_cursor INTO v_emp_id, v_emp_name;
    
    EXIT WHEN emp_cursor%NOTFOUND;
    
    DBMS_OUTPUT.PUT_LINE('Employee ID: ' || v_emp_id || ', Name: ' || v_emp_name);
    DBMS_OUTPUT.PUT_LINE('Total rows fetched: ' || emp_cursor%ROWCOUNT);
  END LOOP;

  CLOSE emp_cursor;
END;

Explanation of the Example

In this example:

• We open the emp_cursor and use a loop to fetch employee details.
• After each successful fetch, we output the total number of rows fetched using %ROWCOUNT.
• The loop continues until no more rows are found, as indicated by %NOTFOUND.

Combining Cursor Attributes

You can combine cursor attributes to create robust PL/SQL programs that handle different scenarios effectively. Here’s an example that uses all three cursor attributes:

Example of Using All Cursor Attributes

DECLARE
  CURSOR emp_cursor IS
    SELECT employee_id, employee_name FROM employees;
  
  v_emp_id employees.employee_id%TYPE;
  v_emp_name employees.employee_name%TYPE;
  v_row_count INTEGER := 0;  -- Initialize a row count variable
BEGIN
  OPEN emp_cursor;
  
  LOOP
    FETCH emp_cursor INTO v_emp_id, v_emp_name;
    
    IF emp_cursor%NOTFOUND THEN
      DBMS_OUTPUT.PUT_LINE('No more employees to fetch.');
      EXIT;
    END IF;

    DBMS_OUTPUT.PUT_LINE('Employee ID: ' || v_emp_id || ', Name: ' || v_emp_name);
    v_row_count := emp_cursor%ROWCOUNT;  -- Update the row count
  END LOOP;

  DBMS_OUTPUT.PUT_LINE('Total employees fetched: ' || v_row_count);
  
  CLOSE emp_cursor;
END;

Explanation of the Example

In this combined example:

• We declare an explicit cursor emp_cursor to fetch employee details.
• We initialise a v_row_count variable to keep track of the number of rows fetched.
• The loop fetches employee details until no more rows are found.
• After each fetch, we update the v_row_count variable with %ROWCOUNT.
• Finally, we print the total number of employees fetched after exiting the loop.

Advantages of PL/SQL Cursor Attributes

Cursor Attributes in PL/SQL provide valuable information about the status of a cursor, allowing developers to handle cursor operations more effectively. These attributes enhance the usability and functionality of cursors in PL/SQL applications. Here are the key advantages of using PL/SQL Cursor Attributes:

1. Enhanced Status Monitoring

Cursor attributes such as %FOUND, %NOTFOUND, %ROWCOUNT, and %ISOPEN enable developers to easily monitor the status of a cursor. This capability allows for more effective control over cursor behavior based on the number of rows fetched or the success of a cursor operation.

2. Simplified Code Logic

By utilizing cursor attributes, developers can simplify code logic and reduce the number of explicit checks required. For example, checking whether a cursor has returned any rows can be done with %FOUND, eliminating the need for additional queries or complex conditions.

3. Improved Error Handling

Cursor attributes help in error handling by providing immediate feedback on the result of cursor operations. For instance, using %NOTFOUND can help in determining if a fetch operation was successful, enabling developers to handle errors gracefully and avoid unexpected behavior.

4. Dynamic Control Flow

Cursor attributes facilitate dynamic control flow in PL/SQL programs. Developers can implement logic that reacts to the number of rows processed or the state of the cursor, allowing for more adaptive and responsive code structures.

5. Reduced Complexity in Fetching Data

Using cursor attributes simplifies the process of fetching data. Developers can easily determine if a cursor is open or if it has fetched any rows, allowing for streamlined logic and fewer redundant operations.

6. Better Resource Management

The ability to check if a cursor is open using the %ISOPEN attribute helps prevent resource leaks by ensuring that cursors are only closed when they are indeed open. This careful resource management contributes to overall application performance and stability.

7. Facilitated Loop Control

Cursor attributes can be particularly useful in loop constructs. For instance, using %ROWCOUNT within a loop allows developers to keep track of how many records have been processed, which can be valuable for logging or conditional logic.

8. Enhanced Readability

Incorporating cursor attributes into PL/SQL code can enhance readability and maintainability. By clearly indicating the status and results of cursor operations, developers can make their code easier to understand for future maintenance and updates.

9. Efficient Data Processing

With attributes like %ROWCOUNT, developers can implement efficient data processing strategies, such as breaking out of loops after a certain number of records have been processed, optimizing performance and resource usage.

10. Useful for Debugging

Cursor attributes provide critical information that can aid in debugging. By checking cursor attributes, developers can quickly identify issues related to cursor operations, such as unexpected row counts or fetch failures, facilitating quicker resolution of bugs.

Disadvantages of PL/SQL Cursor Attributes

While Cursor Attributes in PL/SQL offer numerous benefits, they also come with certain disadvantages that developers should consider. Understanding these drawbacks can help ensure the effective use of cursor attributes in PL/SQL programming. Here are the key disadvantages of using PL/SQL Cursor Attributes:

1. Limited Information

Cursor attributes provide only basic information about the cursor’s state (e.g., whether it is open, how many rows were fetched). This limited scope means that more complex conditions or detailed insights into cursor behavior may require additional logic or processing.

2. Increased Complexity in Multi-Cursor Scenarios

In applications that utilize multiple cursors, managing cursor attributes can become complex. Developers must track the attributes of several cursors simultaneously, which can lead to confusion and errors if not handled carefully.

3. Potential for Misinterpretation

Developers may misinterpret the meanings of cursor attributes, especially if they are not well-versed in PL/SQL. For instance, relying solely on %NOTFOUND without additional checks can lead to unexpected outcomes if not understood in context.

4. Overhead in Performance

While cursor attributes can simplify coding, excessive reliance on them may introduce performance overhead. For example, frequent checks of cursor attributes within loops can slow down execution, particularly if there are many iterations.

5. Dependency on Cursor Lifecycle

Cursor attributes depend on the cursor lifecycle, meaning they can only be used effectively if the cursor is correctly managed (i.e., opened, fetched, and closed appropriately). Poor management can lead to inaccurate attribute values or runtime errors.

6. Limited Error Handling Capabilities

Cursor attributes can signal basic states (like whether rows were found), but they do not provide detailed error handling capabilities. Developers may still need to implement additional error-handling logic for more complex scenarios.

7. Readability Challenges in Complex Logic

In scenarios with intricate cursor logic, the use of multiple cursor attributes can complicate readability. Developers may find it harder to follow the logic flow if many attributes are being checked in various places in the code.

8. Lack of Flexibility

Cursor attributes are static in nature and may not adapt well to dynamic SQL scenarios. This limitation means developers must often combine cursor attributes with other techniques to achieve the desired flexibility.

9. Potential for Resource Leaks

Relying on cursor attributes without proper management can lead to resource leaks, especially if attributes are checked without ensuring the cursor is properly opened and closed.

10. Not Suitable for All Scenarios

While cursor attributes are useful in many cases, they may not be suitable for all scenarios. Certain applications or complex data processing tasks may require more advanced techniques that cursor attributes cannot provide.