Declaring and Using Cursors in Proc*C

Cursors are an essential feature of Proc*C, enabling you to process query results row by row in database-centric C applications. A cursor provides a way to retrieve and manipulate multiple rows returned by an SQL query. In this tutorial, we will explore how to effectively declare and use cursors in Proc*C, with examples and step-by-step explanations to illustrate their significance in building robust and efficient database operations.

1. Declaring Cursors

To declare a cursor in Proc*C, you need to use the EXEC SQL DECLARE statement. A cursor declaration specifies the SQL query that the cursor will execute and the host variables that will hold the data retrieved from the query.

      /* EXEC SQL BEGIN DECLARE SECTION; */
      int employee_id;
      char employee_name[50];
      /* EXEC SQL END DECLARE SECTION; */
  /* Declare the cursor */
  /* EXEC SQL DECLARE emp_cursor CURSOR FOR
           SELECT employee_id, employee_name FROM employees
           WHERE department_id = 100; */

In this example, we declare a cursor named emp_cursor to retrieve employee_id and employee_name from the employees table based on a specific department_id value (100 in this case).

2. Opening and Fetching from Cursors

After declaring a cursor, you can open it using the EXEC SQL OPEN statement, and then fetch rows from the cursor one by one using the EXEC SQL FETCH statement.

      /* EXEC SQL OPEN emp_cursor; */
  /* Fetch rows from the cursor */
  while (SQLCODE == 0) {
      /* EXEC SQL FETCH emp_cursor INTO :employee_id, :employee_name; */
      printf("Employee ID: %d, Employee Name: %s\n", employee_id, employee_name);
  }

  /* EXEC SQL CLOSE emp_cursor; */

In this example, we open the emp_cursor and then use a loop to fetch each row from the cursor. The values of employee_id and employee_name are updated with each fetch, and we print them to the console.

3. Common Mistakes with Cursors in Proc*C

• Forgetting to open the cursor before fetching rows.
• Not checking the SQLCODE after each fetch, leading to infinite loops or incorrect results.
• Using incorrect host variable data types in the cursor declaration.

4. Frequently Asked Questions (FAQs)

• Q: Can I use cursors for all types of SQL queries?
  A: Yes, cursors can be used with any SELECT statement that returns multiple rows.
• Q: How do I handle errors while using cursors?
  A: You can check the SQLCODE after each cursor operation and handle errors accordingly using error handling techniques.
• Q: Can I use cursors to update or delete rows in the database?
  A: Yes, you can use cursors with UPDATE and DELETE statements to modify or remove rows from the database.
• Q: Can I declare multiple cursors in a single Proc*C program?
  A: Yes, you can declare and use multiple cursors in a single program to perform various database operations.
• Q: Is there a performance impact of using cursors?
  A: Cursors may have a slight performance overhead compared to single-row queries, but their benefits in handling multiple rows outweigh this in most cases.

5. Summary

Declaring and using cursors in Proc*C is crucial for efficiently handling multiple rows returned by SQL queries. By declaring the cursor, opening it, and then fetching rows one by one, you can process query results effectively in your database-centric C applications. Avoid common mistakes and refer to the FAQs for any queries related to cursors. With this understanding, you can utilize cursors to create robust and efficient Proc*C programs that interact seamlessly with the database.