Cursors for row iteration in MySQL - Time & Space Complexity

Time Complexity: Cursors for row iteration

O(n)

Understanding Time Complexity

When using cursors in MySQL, we want to understand how the time to process rows grows as the number of rows increases.

We ask: How does the work change when we have more rows to go through one by one?

Scenario Under Consideration

Analyze the time complexity of the following cursor usage in MySQL.

DECLARE done INT DEFAULT FALSE;
DECLARE cur CURSOR FOR SELECT id FROM employees;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;

OPEN cur;
read_loop: LOOP
  FETCH cur INTO @emp_id;
  IF done THEN
    LEAVE read_loop;
  END IF;
  -- Process each employee id here
END LOOP;
CLOSE cur;
    

This code opens a cursor to go through each employee's id one by one and processes them.

Identify Repeating Operations

Look for repeated steps in the code.

• Primary operation: Fetching each row from the cursor and processing it.
• How many times: Once for every row in the employees table.

How Execution Grows With Input

As the number of rows grows, the number of fetch and process steps grows the same way.

Input Size (n)	Approx. Operations
10	About 10 fetch and process steps
100	About 100 fetch and process steps
1000	About 1000 fetch and process steps

Pattern observation: The work grows directly in proportion to the number of rows.

Final Time Complexity

Time Complexity: O(n)

This means the time to complete grows linearly with the number of rows processed.

Common Mistake

[X] Wrong: "Using a cursor is like running one query, so it always takes the same time regardless of rows."

[OK] Correct: Each row is fetched and processed one at a time, so more rows mean more steps and more time.

Interview Connect

Understanding how cursors work and their time cost helps you explain row-by-row processing clearly and shows you know how database operations scale.

Self-Check

What if we replaced the cursor with a single query that processes all rows at once? How would the time complexity change?

Practice

1. What is the main purpose of using a cursor in MySQL?

easy

A. To create new tables dynamically

B. To speed up bulk inserts

C. To process query results row by row

D. To backup the database automatically

Solution

1. Step 1: Understand cursor functionality

   Cursors allow you to handle each row of a query result individually instead of all at once.

2. Step 2: Identify the correct purpose

   Processing rows one by one matches the cursor's main use, unlike speeding inserts or creating tables.

3. Final Answer:

   To process query results row by row -> Option C

4. Quick Check:

   Cursor = row-by-row processing [OK]

Hint: Cursors = row-by-row handling, not bulk operations [OK]

Common Mistakes:

• Thinking cursors speed up inserts
• Confusing cursors with backup tools
• Assuming cursors create tables

2. Which of the following is the correct order of cursor operations in MySQL?

easy

A. DECLARE, FETCH, OPEN, CLOSE

B. DECLARE, OPEN, FETCH, CLOSE

C. FETCH, DECLARE, OPEN, CLOSE

D. OPEN, DECLARE, FETCH, CLOSE

Solution

1. Step 1: Recall cursor lifecycle

   First, you declare the cursor, then open it to start fetching rows, and finally close it when done.

2. Step 2: Match the correct sequence

   Only DECLARE, OPEN, FETCH, CLOSE follows the correct order of cursor operations.

3. Final Answer:

   DECLARE, OPEN, FETCH, CLOSE -> Option B

4. Quick Check:

   Cursor order = DECLARE -> OPEN -> FETCH -> CLOSE [OK]

Hint: Remember: Declare first, then open, fetch, and close last [OK]

Common Mistakes:

• Opening cursor before declaring
• Fetching before opening
• Closing before fetching all rows

3. Consider this MySQL cursor code snippet inside a stored procedure:

DECLARE done INT DEFAULT FALSE;
DECLARE cur CURSOR FOR SELECT id FROM users;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
OPEN cur;
read_loop: LOOP
  FETCH cur INTO @user_id;
  IF done THEN
    LEAVE read_loop;
  END IF;
  SELECT @user_id;
END LOOP;
CLOSE cur;

What will this code output?

medium

A. It prints each user id from the users table one by one

B. It prints only the first user id

C. It causes a syntax error due to missing variable declaration

D. It prints nothing because the loop never runs

Solution

1. Step 1: Analyze cursor fetching loop

   The cursor fetches each id from users table into @user_id, then prints it until no rows remain.

2. Step 2: Understand handler and loop exit

   The handler sets done = TRUE when no more rows, causing loop to exit properly after printing all ids.

3. Final Answer:

   It prints each user id from the users table one by one -> Option A

4. Quick Check:

   Cursor loop prints all rows one by one [OK]

Hint: Handler sets done; loop prints all fetched rows [OK]

Common Mistakes:

• Assuming only first row prints
• Missing handler causes infinite loop
• Confusing session variables with local variables

4. You wrote this MySQL cursor code but it causes an infinite loop:

DECLARE done INT DEFAULT FALSE;
DECLARE cur CURSOR FOR SELECT name FROM employees;
OPEN cur;
read_loop: LOOP
  FETCH cur INTO @emp_name;
  IF done THEN
    LEAVE read_loop;
  END IF;
  SELECT @emp_name;
END LOOP;
CLOSE cur;

What is the likely cause of the infinite loop?

medium

A. The SELECT statement inside loop causes the loop to restart

B. Cursor was not declared before opening

C. Variable @emp_name is not declared

D. Missing CONTINUE HANDLER to set done = TRUE when no rows left

Solution

1. Step 1: Check for handler presence

   The code lacks a CONTINUE HANDLER to set done = TRUE when no more rows are fetched.

2. Step 2: Understand loop exit condition

   Without setting done = TRUE, the IF condition never triggers LEAVE, causing infinite loop.

3. Final Answer:

   Missing CONTINUE HANDLER to set done = TRUE when no rows left -> Option D

4. Quick Check:

   No handler = infinite loop [OK]

Hint: Always add handler to set done flag for loop exit [OK]

Common Mistakes:

• Forgetting CONTINUE HANDLER
• Assuming loop exits automatically
• Misplacing variable declarations

5. You want to write a stored procedure that sums the salaries of all employees using a cursor. Which approach correctly uses a cursor to calculate the total salary?

DECLARE done INT DEFAULT FALSE;
DECLARE total_salary DECIMAL(10,2) DEFAULT 0;
DECLARE cur CURSOR FOR SELECT salary FROM employees;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
OPEN cur;
read_loop: LOOP
  FETCH cur INTO @sal;
  IF done THEN
    LEAVE read_loop;
  END IF;
  SET total_salary = total_salary + @sal;
END LOOP;
CLOSE cur;
SELECT total_salary;

What is the expected output of this procedure?

hard

A. The sum of all employee salaries

B. Syntax error due to missing variable declaration for @sal

C. Zero, because total_salary is not updated correctly

D. The procedure runs but returns NULL

Solution

1. Step 1: Verify declaration order and variables

   All local variables (done, total_salary) are declared before the cursor and handler. FETCH INTO @sal uses a session variable, which is valid.

2. Step 2: Trace execution

   The loop fetches each salary, adds it to total_salary until the handler sets done=TRUE, then SELECT outputs the sum.

3. Final Answer:

   The sum of all employee salaries -> Option A

4. Quick Check:

   Cursor loop + handler + accumulation = total sum [OK]

Hint: DECLARE order: locals first, then cursors, handlers. FETCH INTO @var OK [OK]

Common Mistakes:

• Declaring variables after cursors/handlers
• Thinking @vars require DECLARE
• Missing handler or wrong loop condition