What are PL/SQL Cursors, and why are they used?

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Vinay Kumar

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Nov 03, 2024 5:30 am

(@Vinay Kumar)

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In PL/SQL, a cursor is a pointer to a context area on the database, which allows you to retrieve and manipulate query results row by row. Cursors are essential in PL/SQL for handling SQL query results, especially when the query returns multiple rows. They help manage the row-by-row processing of data effectively within PL/SQL blocks.

1. Types of Cursors in PL/SQL

PL/SQL supports two main types of cursors:

Implicit Cursors: These are automatically created by Oracle when a single-row SQL query (such as SELECT INTO) is executed within PL/SQL. The database handles the cursor internally, so you don’t need to declare or manage it.
Explicit Cursors: These are defined explicitly by the programmer and are used when you need to process multiple rows from a query. They provide more control over query execution and data processing.

2. Using an Explicit Cursor

Explicit cursors follow four main steps:

Declare the Cursor: Define the SQL query for the cursor.
Open the Cursor: Open the cursor to execute the query and establish the context.
Fetch Data: Retrieve rows one by one using the FETCH command.
Close the Cursor: Close the cursor to release the context area and free resources.

Here’s a basic example of using an explicit cursor to retrieve employee details:

plsql

DECLARE

    CURSOR emp_cursor IS

        SELECT employee_id, name, salary FROM employees WHERE department_id = 10;

    emp_id employees.employee_id%TYPE;

    emp_name employees.name%TYPE;

    emp_salary employees.salary%TYPE;

BEGIN

    OPEN emp_cursor;

    LOOP

        FETCH emp_cursor INTO emp_id, emp_name, emp_salary;

        EXIT WHEN emp_cursor%NOTFOUND;

        DBMS_OUTPUT.PUT_LINE('ID: ' || emp_id || ' Name: ' || emp_name || ' Salary: ' || emp_salary);

    END LOOP;

    CLOSE emp_cursor;

END;

Explanation:

Declare: The emp_cursor cursor is defined with a query selecting employees from a specific department.
Open: The cursor is opened, executing the query and setting up the result context.
Fetch: Each row is fetched in the loop, assigning data to the variables.
Close: The cursor is closed once all rows are processed, freeing resources.

3. Cursor Attributes

Cursors have built-in attributes that provide information about query execution:

%FOUND: Returns TRUE if the last fetch returned a row, else FALSE.
%NOTFOUND: Returns TRUE if the last fetch did not return a row.
%ROWCOUNT: Returns the number of rows fetched so far.
%ISOPEN: Returns TRUE if the cursor is open.

Using attributes can help manage cursor processing more effectively.

4. Benefits of Using Cursors

Efficient Data Handling: Allows row-by-row processing, which is useful for operations requiring sequential data handling.
Flexibility in Data Retrieval: Provides granular control over query execution and allows custom processing for each row.
Memory Management: Explicitly closing cursors helps manage memory usage effectively.

5. Implicit vs. Explicit Cursors

Implicit Cursors: Simpler and automatic but limited to single-row operations. Ideal for quick SELECT INTO operations.
Explicit Cursors: Offer greater control, supporting multi-row processing and suitable for complex data handling scenarios.

Conclusion

PL/SQL cursors are essential for row-wise data processing in database operations. While implicit cursors are automatically managed for simple queries, explicit cursors provide full control over multi-row data handling. Using explicit cursors along with cursor attributes like %FOUND and %ROWCOUNT enables efficient and flexible data retrieval in PL/SQL. For further insights, check out the Oracle Cursor Documentation for advanced cursor techniques.