Variables and control flow in MySQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of variable assignment and IF statement

What is the output of the following MySQL code snippet?

MySQL

SET @x = 5;
IF @x > 3 THEN
  SELECT 'Greater';
ELSE
  SELECT 'Smaller';
END IF;

A Smaller

B Syntax error

C Greater

D NULL

💡 Hint

Control flow statements like IF require stored routine context.

IF statements can only be used within stored routines (procedures, functions, etc.). This standalone code causes a syntax error.

❓ query_result

intermediate

Output of WHILE loop with variable increment

What will be the output of this MySQL code?

MySQL

SET @count = 1;
SET @result = 0;
WHILE @count <= 3 DO
  SET @result = @result + @count;
  SET @count = @count + 1;
END WHILE;
SELECT @result;

A Syntax error

B 3

C 0

D 6

💡 Hint

Control flow statements like WHILE require stored routine context.

WHILE loops can only be used within stored programs; this standalone code results in a syntax error.

📝 Syntax

advanced

Identify the syntax error in variable declaration

Which option contains the correct syntax to declare and set a variable in MySQL stored procedure?

A DECLARE x INT = 10;

B SET x = 10;

C DECLARE x INT DEFAULT 10;

D DECLARE x = INT 10;

💡 Hint

Look at the correct order of keywords in DECLARE.

In MySQL stored procedures, variables are declared with 'DECLARE var_name type DEFAULT value;'. Options C and D have wrong syntax. Option C sets a variable but does not declare it.

❓ optimization

advanced

Optimize variable usage in a loop

Which option optimizes the following code to avoid unnecessary variable assignments? Original code: SET @i = 1; WHILE @i <= 5 DO SET @sum = @sum + @i; SET @i = @i + 1; END WHILE; SELECT @sum;

A Initialize @sum to 0 before the loop and keep the rest same.

B Remove the loop and directly set @sum = 15;

C Initialize @sum inside the loop to 0 each time.

D Use a FOR loop instead of WHILE.

💡 Hint

Uninitialized variables can cause unexpected results.

Without initializing @sum to 0, the addition may produce NULL or wrong results. Initializing @sum before the loop is necessary.

🧠 Conceptual

expert

Understanding variable scope in MySQL stored procedures

Consider a stored procedure where a variable is declared inside a BEGIN...END block. Which statement is true about the variable's scope?

A The variable is accessible throughout the entire stored procedure regardless of where declared.

B The variable is accessible only after the block ends.

C The variable is global and accessible outside the stored procedure.

D The variable is accessible only within the BEGIN...END block where it is declared.

💡 Hint

Think about local vs global variable scope.

Variables declared inside a BEGIN...END block are local to that block and cannot be accessed outside it.

Practice

1. What is the purpose of a variable in MySQL stored procedures?

easy

A. To create new tables dynamically

B. To permanently save data in the database

C. To store temporary data during procedure execution

D. To define user permissions

Solution

1. Step 1: Understand variable role in procedures

   Variables hold temporary data only while the procedure runs.

2. Step 2: Differentiate from permanent storage

   Permanent data is stored in tables, not variables.

3. Final Answer:

   To store temporary data during procedure execution -> Option C

4. Quick Check:

   Variables = Temporary data [OK]

Hint: Variables hold temporary data inside procedures [OK]

Common Mistakes:

• Confusing variables with table data storage
• Thinking variables create database objects
• Assuming variables control user access

2. Which of the following is the correct way to declare and set a variable in MySQL stored procedures?

easy

A. SET x INT; DECLARE x = 10;

B. DECLARE x INT; SET x = 10;

C. DECLARE x = 10 INT;

D. SET x = 10; DECLARE x INT;

Solution

1. Step 1: Recall syntax for variable declaration

   Use DECLARE variable_name data_type; to declare.

2. Step 2: Assign value using SET

   Use SET variable_name = value; after declaration.

3. Final Answer:

   DECLARE x INT; SET x = 10; -> Option B

4. Quick Check:

   DECLARE then SET = correct syntax [OK]

Hint: Declare first, then set value with SET [OK]

Common Mistakes:

• Using SET before DECLARE
• Combining declaration and assignment incorrectly
• Wrong order of keywords

3. Consider the following MySQL procedure snippet:

DECLARE counter INT DEFAULT 1;
WHILE counter <= 3 DO
  SET counter = counter + 1;
END WHILE;
SELECT counter;

What will be the output of the SELECT counter; statement?

medium

A. 3

B. Error due to missing BEGIN...END

C. 1

D. 4

Solution

1. Step 1: Trace the WHILE loop increments

   Counter starts at 1, increments by 1 each loop until counter > 3.

2. Step 2: Count loop iterations and final value

   Loop runs while counter=1,2,3; increments to 4 and stops.

3. Final Answer:

   4 -> Option D

4. Quick Check:

   Counter ends at 4 after loop [OK]

Hint: WHILE increments counter until condition false [OK]

Common Mistakes:

• Assuming counter stops at 3
• Forgetting counter increments before condition check
• Thinking syntax error without BEGIN...END in snippet

4. Identify the error in this MySQL stored procedure snippet:

DECLARE total INT;
SET total = 0
WHILE total < 5 DO
  SET total = total + 1;
END WHILE;

medium

A. Missing semicolon after SET total = 0

B. WHILE loop condition syntax is wrong

C. DECLARE must be inside BEGIN...END block

D. Variable total not declared

Solution

1. Step 1: Check statement endings

   SET total = 0 is missing a semicolon at the end.

2. Step 2: Validate syntax for DECLARE and WHILE

   DECLARE is valid before BEGIN, and WHILE syntax is correct.

3. Final Answer:

   Missing semicolon after SET total = 0 -> Option A

4. Quick Check:

   Every statement needs semicolon [OK]

Hint: Check semicolons after each statement [OK]

Common Mistakes:

• Ignoring missing semicolon errors
• Thinking DECLARE must be inside BEGIN...END always
• Misreading WHILE syntax

5. You want to create a MySQL stored procedure that counts how many numbers between 1 and 10 are even, using variables and control flow. Which of the following code snippets correctly implements this?

hard

A. DECLARE count_even INT DEFAULT 0; DECLARE i INT DEFAULT 1; WHILE i <= 10 DO IF i % 2 = 0 THEN SET count_even = count_even + 1; END IF; SET i = i + 1; END WHILE; SELECT count_even;

B. DECLARE count_even INT; SET count_even = 0; FOR i IN 1..10 DO IF i % 2 = 0 THEN SET count_even = count_even + 1; END IF; END FOR; SELECT count_even;

C. DECLARE count_even INT DEFAULT 0; SET i = 1; WHILE i < 10 DO IF i % 2 = 1 THEN SET count_even = count_even + 1; END IF; SET i = i + 1; END WHILE; SELECT count_even;

D. DECLARE count_even INT DEFAULT 0; DECLARE i INT DEFAULT 1; WHILE i <= 10 DO IF i / 2 = 0 THEN SET count_even = count_even + 1; END IF; SET i = i + 1; END WHILE; SELECT count_even;

Solution

1. Step 1: Check loop and variable initialization

   DECLARE count_even INT DEFAULT 0; DECLARE i INT DEFAULT 1; WHILE i <= 10 DO IF i % 2 = 0 THEN SET count_even = count_even + 1; END IF; SET i = i + 1; END WHILE; SELECT count_even; correctly declares variables and initializes them.

2. Step 2: Verify condition and increment logic

   DECLARE count_even INT DEFAULT 0; DECLARE i INT DEFAULT 1; WHILE i <= 10 DO IF i % 2 = 0 THEN SET count_even = count_even + 1; END IF; SET i = i + 1; END WHILE; SELECT count_even; uses WHILE loop from 1 to 10, checks even with modulo, increments count and i properly.

3. Step 3: Identify errors in other options

   DECLARE count_even INT; SET count_even = 0; FOR i IN 1..10 DO IF i % 2 = 0 THEN SET count_even = count_even + 1; END IF; END FOR; SELECT count_even; uses invalid FOR syntax; C counts odd numbers; D uses division instead of modulo.

4. Final Answer:

   Option A code correctly counts even numbers between 1 and 10 -> Option A

5. Quick Check:

   Use WHILE + modulo for even check [OK]

Hint: Use WHILE loop and modulo (%) to check even numbers [OK]

Common Mistakes:

• Using invalid FOR loop syntax in MySQL
• Confusing modulo (%) with division (/)
• Counting odd numbers instead of even