In DB2, what is a cursor and why would you use a cursor when working with query results in an application program?

Introduction / Context:
When an application program interacts with a DB2 database, it often needs to process the rows returned by a SELECT statement. In many cases, it is not practical or necessary to fetch all rows at once. Instead, the program may process one row at a time, performing calculations or updates along the way. DB2 cursors provide a structured way to handle this row by row processing. Understanding what a cursor is and why it is used is important for anyone working with embedded SQL or host language programs in DB2.

Given Data / Assumptions:
A SELECT statement can return multiple rows in a result set.Application programs written in languages such as COBOL, C, or Java need to retrieve these rows for processing.DB2 supports the concept of a cursor for controlled navigation through a result set.The question asks both for a definition of a cursor and its purpose.

Concept / Approach:
A cursor in DB2 is a named control structure associated with the result of a query. The program declares the cursor, opens it to execute the associated SELECT, and then repeatedly fetches rows from the cursor. Each FETCH moves to the next row and returns its column values to host variables. Some cursors can be defined as updatable, allowing the program to issue positioned UPDATE or DELETE statements against the current row. When processing is complete, the program closes the cursor. This mechanism lets applications handle large result sets in a memory efficient, sequential manner.

Step-by-Step Solution:
First, recall that a SELECT statement may return many rows, and an application often needs to process them one by one.Next, understand that a cursor is declared in the application code and associated with a SELECT statement that defines the result set.Then, recognize that OPEN cursor executes the SELECT, and FETCH cursor retrieves the next row each time it is called, advancing the cursor position.After that, note that cursors can be used with positioned UPDATE and DELETE operations to modify the current row in the result set if needed.Finally, compare the options and see that option A captures this definition and purpose, while the other options describe unrelated hardware or tools.

Verification / Alternative check:
DB2 manuals describe cursors as control structures that enable row at a time processing of query results. They outline the typical sequence: DECLARE CURSOR, OPEN, FETCH, optionally positioned UPDATE or DELETE, and CLOSE. These documents emphasize that cursors are essential when the host language cannot naturally process a multi row result in a single statement. None of the documentation treats cursors as hardware devices, indexes that remove duplicates, backup utilities, or log files. This confirms the correctness of option A.

Why Other Options Are Wrong:
Option B humorously refers to a mouse device, which is unrelated to DB2 cursor objects. Option C incorrectly describes a cursor as an index that removes duplicates, which confusion belongs to different database features. Option D associates cursors with backups, but backup tools and utilities are separate from query processing. Option E calls a cursor a log file, whereas logs are used for recovery and auditing, not for row by row navigation of a result set.

Common Pitfalls:
Developers sometimes overuse cursors for tasks that could be done more efficiently with set based SQL operations, leading to slower programs. Another pitfall is forgetting to close cursors, which can hold resources and cause performance issues. In some cases, poorly designed updatable cursors can lead to locking problems. The best practice is to use cursors when truly necessary for procedural logic, while preferring set based queries whenever possible. Knowing exactly what a cursor is and when to use it helps strike this balance.

Final Answer:
The correct answer is: A cursor is a database object that provides a controlled, row by row mechanism for traversing a result set so that an application can fetch, process, and possibly update one row at a time.