In this section, we will delve into the concept of cursors in COBOL, particularly when working with databases like DB2. Cursors are essential for handling multiple rows of data retrieved from a database. They allow you to process each row individually, which is crucial for applications that need to handle large datasets.
What is a Cursor?
A cursor is a database object used to retrieve, manipulate, and navigate through a result set row by row. It acts as a pointer to the rows in a result set, enabling you to fetch and process each row sequentially.
Key Concepts of Cursors
- Declaration: Define the cursor and the SQL query it will execute.
- Opening: Execute the SQL query and establish the result set.
- Fetching: Retrieve rows from the result set one at a time.
- Closing: Release the cursor and its resources.
Declaring a Cursor
To declare a cursor in COBOL, you use the EXEC SQL DECLARE CURSOR statement. This statement specifies the SQL query that the cursor will execute.
Example
EXEC SQL
DECLARE EMP_CURSOR CURSOR FOR
SELECT EMP_ID, EMP_NAME, EMP_SALARY
FROM EMPLOYEE
WHERE DEPARTMENT = 'SALES'
END-EXEC.In this example, EMP_CURSOR is declared to select employee details from the EMPLOYEE table where the department is 'SALES'.
Opening a Cursor
After declaring the cursor, you need to open it to execute the SQL query and establish the result set.
Example
Fetching Data
Once the cursor is open, you can fetch rows from the result set. The FETCH statement retrieves the next row and moves the cursor to the subsequent row.
Example
In this example, the FETCH statement retrieves the next row from EMP_CURSOR and stores the values into COBOL variables EMP-ID, EMP-NAME, and EMP-SALARY.
Looping Through Rows
Typically, you will use a loop to fetch all rows from the cursor until no more rows are available.
Example
PERFORM UNTIL SQLCODE NOT = 0
EXEC SQL
FETCH EMP_CURSOR INTO :EMP-ID, :EMP-NAME, :EMP-SALARY
END-EXEC
IF SQLCODE = 0
DISPLAY 'Employee ID: ' EMP-ID
DISPLAY 'Employee Name: ' EMP-NAME
DISPLAY 'Employee Salary: ' EMP-SALARY
END-IF
END-PERFORM.In this example, the loop continues fetching rows until SQLCODE is not equal to 0, indicating no more rows are available.
Closing a Cursor
After processing all rows, you should close the cursor to release its resources.
Example
Practical Exercise
Task
Write a COBOL program that uses a cursor to fetch and display all employees' details from the EMPLOYEE table where the department is 'HR'.
Solution
IDENTIFICATION DIVISION.
PROGRAM-ID. FetchEmployees.
DATA DIVISION.
WORKING-STORAGE SECTION.
01 EMP-ID PIC 9(5).
01 EMP-NAME PIC X(20).
01 EMP-SALARY PIC 9(7)V99.
EXEC SQL
INCLUDE SQLCA
END-EXEC.
PROCEDURE DIVISION.
MAIN-LOGIC.
EXEC SQL
DECLARE HR_CURSOR CURSOR FOR
SELECT EMP_ID, EMP_NAME, EMP_SALARY
FROM EMPLOYEE
WHERE DEPARTMENT = 'HR'
END-EXEC.
EXEC SQL
OPEN HR_CURSOR
END-EXEC.
PERFORM UNTIL SQLCODE NOT = 0
EXEC SQL
FETCH HR_CURSOR INTO :EMP-ID, :EMP-NAME, :EMP-SALARY
END-EXEC
IF SQLCODE = 0
DISPLAY 'Employee ID: ' EMP-ID
DISPLAY 'Employee Name: ' EMP-NAME
DISPLAY 'Employee Salary: ' EMP-SALARY
END-IF
END-PERFORM.
EXEC SQL
CLOSE HR_CURSOR
END-EXEC.
STOP RUN.Explanation
- Declare Cursor: The cursor
HR_CURSORis declared to select employee details from theEMPLOYEEtable where the department is 'HR'. - Open Cursor: The cursor is opened to execute the SQL query.
- Fetch Rows: A loop fetches each row from the cursor and displays the employee details.
- Close Cursor: The cursor is closed after all rows are processed.
Common Mistakes and Tips
- Forgetting to Close the Cursor: Always close the cursor to free up resources.
- Handling SQLCODE: Check
SQLCODEafter eachFETCHto handle any errors or end-of-data conditions. - Variable Mismatch: Ensure that the COBOL variables match the data types and lengths of the database columns.
Conclusion
In this section, you learned how to handle cursors in COBOL for database operations. Cursors are powerful tools for processing multiple rows of data efficiently. By mastering cursor operations, you can build robust COBOL applications that interact seamlessly with databases. In the next module, we will explore advanced topics in COBOL, including object-oriented programming and web services.