What are SQL cursors, and how are they used?

SQL cursors are database objects used to process individual rows from a result set one at a time, rather than handling data in bulk. They act like pointers that traverse rows returned by a query, allowing developers to perform row-by-row operations, such as updates or complex calculations, that are difficult to achieve with standard set-based SQL operations. Cursors are typically used when iterative processing is necessary, such as applying business logic that depends on values from multiple rows or executing stored procedures for each row. However, they are generally less efficient than set-based operations and should be used sparingly.

To use a cursor, you first declare it using the DECLARE statement, specifying the SQL query that defines the result set. For example, DECLARE employee_cursor CURSOR FOR SELECT id, name FROM employees; creates a cursor for employee data. Next, you open the cursor with OPEN employee_cursor;, which executes the query and prepares the result set. You then fetch rows sequentially using FETCH NEXT FROM employee_cursor INTO @id, @name;, storing column values into variables. This is often done within a loop (e.g., WHILE @@FETCH_STATUS = 0) to process all rows. After completing operations, you close the cursor with CLOSE employee_cursor; and release resources with DEALLOCATE employee_cursor;. For example, a cursor might loop through sales records, calculate taxes for each row, and update a column with the result.

While cursors provide flexibility, they can negatively impact performance due to repeated database roundtrips and resource locks. Set-based operations (e.g., UPDATE employees SET salary = salary * 1.05;) are preferable for bulk data manipulation. Cursors are best suited for tasks that require row-specific logic, such as generating dynamic reports or cascading updates across related tables. When using cursors, opt for efficient types like FAST_FORWARD (read-only, forward-only) to minimize overhead. Always ensure proper error handling and transaction management to avoid leaving cursors open or causing deadlocks. In summary, cursors are a tool for specific scenarios but should be used cautiously after evaluating alternatives.