Cursors for row iteration in MySQL - Step-by-Step Execution

Concept Flow - Cursors for row iteration

Declare Cursor

↓

Open Cursor

↓

Fetch Row

↓

Check if Row Exists?

No→Close Cursor

Yes↓

Process Row

↓

Fetch Next Row

↩Back to Check if Row Exists?

This flow shows how a cursor is declared, opened, and used to fetch and process rows one by one until no more rows remain, then it closes.

Execution Sample

MySQL

DECLARE done INT DEFAULT 0;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;
DECLARE cur CURSOR FOR SELECT id, name FROM users;
OPEN cur;
FETCH cur INTO @id, @name;
WHILE NOT done DO
  -- process row
  FETCH cur INTO @id, @name;
END WHILE;
CLOSE cur;

This code declares a done flag and handler, declares a cursor to select id and name from users, opens it, fetches each row into variables, processes them, and closes the cursor.

Execution Table

Step	Action	Cursor State	Fetched Values	Condition done	Next Step
1	DECLARE done, handler, cursor for SELECT id, name FROM users	Not opened	None	N/A	Open cursor
2	OPEN cursor	Opened, ready to fetch	None	N/A	Fetch first row
3	FETCH first row	Opened	id=1, name='Alice'	0 (row fetched)	Process row
4	Process row (id=1, name='Alice')	Opened	id=1, name='Alice'	0	Fetch next row
5	FETCH next row	Opened	id=2, name='Bob'	0	Process row
6	Process row (id=2, name='Bob')	Opened	id=2, name='Bob'	0	Fetch next row
7	FETCH next row	Opened	id=3, name='Carol'	0	Process row
8	Process row (id=3, name='Carol')	Opened	id=3, name='Carol'	0	Fetch next row
9	FETCH next row	Opened	None	1 (no row)	Close cursor
10	CLOSE cursor	Closed	None	N/A	End

💡 No more rows to fetch, done = 1, cursor closed.

Variable Tracker

Variable	Start	After Step 3	After Step 5	After Step 7	After Step 9
@id	NULL	1	2	3	NULL
@name	NULL	Alice	Bob	Carol	NULL
done	0	0	0	0	1

Key Moments - 3 Insights

Why do we check done after each FETCH?

Because done tells us if the FETCH got a row (0) or not (1). We stop fetching when no rows remain, as shown in steps 3, 5, 7 (0) and step 9 (1).

What happens if we forget to CLOSE the cursor?

The cursor stays open, which can cause resource leaks. Step 10 shows the proper closing after fetching all rows.

Can we process rows before fetching the first one?

No, we must FETCH first to get data. Step 3 fetches the first row before processing in step 4.

Visual Quiz - 3 Questions

Look at the execution table, what are the values of @id and @name after step 5?

A @id=3, @name='Carol'

B @id=1, @name='Alice'

C @id=2, @name='Bob'

D NULL, NULL

💡 Hint

Check the 'Fetched Values' column at step 5 in the execution table.

At which step does the cursor detect no more rows to fetch?

A Step 9

B Step 7

C Step 3

D Step 10

💡 Hint

Look for done = 1 in the execution table.

If we skip the CLOSE cursor step, what is the likely effect?

A Cursor automatically closes after last fetch

B Cursor remains open, wasting resources

C Fetch will fail immediately

D No effect, code runs normally

💡 Hint

Refer to key moment about closing cursor and step 10 in the execution table.

Concept Snapshot

DECLARE done INT DEFAULT 0; DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;
DECLARE cursor FOR SELECT ...;  -- Define cursor
OPEN cursor;  -- Start cursor
FETCH cursor INTO variables;  -- Get one row
WHILE NOT done DO  -- Loop while rows exist
  -- Process row
  FETCH cursor INTO variables;  -- Next row
END WHILE;
CLOSE cursor;  -- Release cursor

Full Transcript

This visual execution shows how a MySQL cursor works step-by-step. First, the done flag and CONTINUE HANDLER are declared, then the cursor is declared for a SELECT query. Then it is opened to start fetching rows. Each FETCH gets one row into variables, and done tells if a row was fetched (0) or not (1). We process each row while NOT done. When no rows remain, done becomes 1, and we close the cursor to free resources. Variables @id and @name hold the current row's data. This process lets us handle rows one by one in SQL procedures.

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