Working with Multiple Result Sets in Proc*C

In database-centric C applications, it is common to retrieve data from the database in the form of result sets. A result set is a set of rows that satisfy a specific query. However, in some scenarios, you may need to retrieve multiple result sets from the database in a single database call. In this tutorial, we will explore how to work with multiple result sets in Proc*C and explain the steps to efficiently handle and process multiple result sets in C applications.

1. Retrieving Multiple Result Sets

In Proc*C, you can retrieve multiple result sets using the EXEC SQL statements. Let's take an example of a simple SQL query that retrieves two result sets from the employees table, one containing employees with a specific job title and another containing employees with a different job title:


          /* EXEC SQL BEGIN DECLARE SECTION; */
      char job_title1[20] = "Manager";
      char job_title2[20] = "Developer";
      /* EXEC SQL END DECLARE SECTION; */
  /* EXEC SQL DECLARE c1 CURSOR FOR
     SELECT * FROM employees WHERE job_title = :job_title1;
  END-EXEC; */

  /* EXEC SQL DECLARE c2 CURSOR FOR
     SELECT * FROM employees WHERE job_title = :job_title2;
  END-EXEC; */

  /* EXEC SQL OPEN c1;
  END-EXEC; */
  // Process result set c1

  /* EXEC SQL CLOSE c1;
  END-EXEC; */

  /* EXEC SQL OPEN c2;
  END-EXEC; */
  // Process result set c2

  /* EXEC SQL CLOSE c2;
  END-EXEC; */

In this example, we declare two character arrays job_title1 and job_title2 to hold the job titles of the employees we want to retrieve. We then declare two cursors c1 and c2 to fetch the result sets based on the job titles. The EXEC SQL OPEN statements are used to open the cursors and retrieve the result sets. After processing each result set, we close the cursors using the EXEC SQL CLOSE statements.

2. Handling Multiple Result Sets

When working with multiple result sets, it's essential to handle each result set separately. After opening a cursor, you can use the appropriate FETCH statements to retrieve the data from the current result set. You can loop through the result set until there are no more rows to fetch. After processing a result set, close the cursor before opening the next one.

3. Common Mistakes with Multiple Result Sets

Not closing the cursor after processing the result set, which may lead to resource leaks.
Fetching data from the wrong cursor or not resetting the cursor position correctly before fetching.
Using incorrect data types or mismatching data types when binding variables for the cursors.

4. Frequently Asked Questions (FAQs)

Q: How many result sets can I retrieve in a single database call?
A: The number of result sets you can retrieve in a single database call depends on the database system and its configuration. Most databases support multiple result sets in a single call.
Q: Can I retrieve different types of result sets in a single database call?
A: Yes, you can retrieve different types of result sets in a single database call, as long as the SQL statements are valid and supported by the database.
Q: How do I know if a cursor contains any rows to fetch?
A: You can use the SQLCODE variable after a fetch operation to check if there are any more rows to fetch. A value of 0 indicates that the fetch was successful, while a positive value indicates that there are more rows to fetch. A negative value indicates an error.
Q: Can I use the same cursor to fetch multiple result sets?
A: No, you need to declare separate cursors for each result set you want to fetch. Attempting to use the same cursor for multiple result sets will result in errors.
Q: Are there any performance considerations when working with multiple result sets?
A: Yes, retrieving and processing multiple result sets can have performance implications. It's essential to optimize your SQL queries and ensure efficient data retrieval to minimize the impact on performance.

5. Summary

Working with multiple result sets in Proc*C allows you to retrieve and process different sets of data efficiently in a single database call. By properly declaring and handling cursors, you can manage multiple result sets seamlessly in your C applications. Be cautious of common mistakes and follow best practices to ensure the successful retrieval and processing of multiple result sets.