SQL cursors are database objects used to retrieve, manipulate, and navigate through a result set row by row. Unlike standard SQL commands that operate on entire sets of data simultaneously, cursors allow more granular control over the data returned by a query. This is particularly useful when performing operations that require processing each row individually, such as complex calculations or updates based on specific conditions. Cursors are mainly used in stored procedures and scripts where iterative processing is needed.
Using a cursor typically involves several steps: declaring the cursor, opening it to establish the result set, fetching rows one at a time for processing, and finally closing and deallocating the cursor when done. For instance, in a scenario where you need to update employee salaries in a database based on certain criteria, you might declare a cursor to select employees meeting those criteria, then loop through the result set to adjust each salary accordingly. Here is a simple example in T-SQL:
DECLARE employee_cursor CURSOR FOR
SELECT EmployeeID, Salary FROM Employees WHERE PerformanceRating > 4;
OPEN employee_cursor;
FETCH NEXT FROM employee_cursor INTO @EmployeeID, @Salary;
WHILE @@FETCH_STATUS = 0
BEGIN
SET @NewSalary = @Salary * 1.10; -- Give a 10% raise
UPDATE Employees SET Salary = @NewSalary WHERE EmployeeID = @EmployeeID;
FETCH NEXT FROM employee_cursor INTO @EmployeeID, @Salary;
END;
CLOSE employee_cursor;
DEALLOCATE employee_cursor;
While cursors offer essential capabilities for row-by-row processing, they can be less efficient than set-based operations. This inefficiency stems from the fact that cursors often require more resources and can lead to increased locking and performance bottlenecks, especially with large datasets. Therefore, it’s recommended to use them selectively, considering alternatives like temporary tables or set-based queries when possible. Cursors can add clarity and control in specific scenarios, but developers should always weigh the pros and cons before opting to use them.