What is a cursor in a relational database?

A cursor in a relational database is a control structure that allows row-by-row processing of the result set obtained from an SQL query.

Main characteristics:

• Iterative execution: The cursor allows sequential movement through the result set, fetching one row at a time for processing.
• State: The cursor maintains the current position within the result set.
• Control: It provides commands to open, fetch the next row, update or delete the current row, and close.

Typical use cases:

• Row-by-row processing of large data sets.
• Performing operations that cannot be efficiently implemented with standard DML operations (INSERT, UPDATE, DELETE).
• Working with stored procedures and functions that require row-level data access.

Example pseudocode (syntax may vary depending on the DBMS):

-- Declare cursor
DECLARE employee_cursor CURSOR FOR
    SELECT employee_id, salary
    FROM employees
    WHERE department = 'IT';

-- Open cursor
OPEN employee_cursor;

-- Fetch first row
FETCH NEXT FROM employee_cursor INTO @emp_id, @emp_salary;

-- Loop through rows
WHILE @@FETCH_STATUS = 0
BEGIN
    -- Process current row (e.g., increase salary)
    UPDATE employees
    SET salary = salary * 1.10
    WHERE employee_id = @emp_id; -- Update current row

    -- Fetch next row
    FETCH NEXT FROM employee_cursor INTO @emp_id, @emp_salary;
END;

-- Close cursor
CLOSE employee_cursor;

-- Deallocate cursor (if necessary)
DEALLOCATE employee_cursor;

Important notes:

• Cursors can be resource-intensive and less efficient than set-based operations, as they process data row by row.
• Use cursors sparingly, favoring set-based operations for better performance.
• There are different types of cursors (static, dynamic, keyset-driven, forward-only) with varying behavior when data changes during cursor operation.