Variables and control flow in MySQL

Introduction

Variables let you store values to use later. Control flow helps you decide what to do next based on conditions.

You want to store a temporary value during a query or procedure.

You need to run different commands depending on some condition.

You want to repeat actions multiple times with loops.

You want to check if a value meets a condition and act accordingly.

You want to organize complex logic inside stored procedures.

Syntax

MySQL

DECLARE variable_name datatype [DEFAULT value];
SET variable_name = value;

IF condition THEN
  -- statements
ELSEIF condition THEN
  -- statements
ELSE
  -- statements
END IF;

WHILE condition DO
  -- statements
END WHILE;

DECLARE is used inside stored programs like procedures or functions.

SET assigns a value to a variable.

IF and WHILE control the flow based on conditions.

Examples

Declare a variable 'count' as an integer and set it to 5.

MySQL

DECLARE count INT DEFAULT 0;
SET count = 5;

Check if 'count' is positive and set 'message' accordingly.

MySQL

IF count > 0 THEN
  SET message = 'Positive';
ELSE
  SET message = 'Zero or Negative';
END IF;

Repeat decreasing 'count' until it reaches zero.

MySQL

WHILE count > 0 DO
  SET count = count - 1;
END WHILE;

Sample Program

This procedure counts down from 3 to 1, sets a message for each number, and shows it.

MySQL

DELIMITER $$
CREATE PROCEDURE demo_variables()
BEGIN
  DECLARE counter INT DEFAULT 3;
  DECLARE result VARCHAR(20);

  WHILE counter > 0 DO
    IF counter = 3 THEN
      SET result = 'Three';
    ELSEIF counter = 2 THEN
      SET result = 'Two';
    ELSE
      SET result = 'One';
    END IF;
    SELECT CONCAT('Count is: ', counter, ', Result is: ', result) AS output;
    SET counter = counter - 1;
  END WHILE;
END$$
DELIMITER ;

CALL demo_variables();

Important Notes

Variables declared with DECLARE only exist inside the procedure or function.

Use DELIMITER to change statement end marker when creating procedures.

Control flow statements must end with END IF, END WHILE, etc.

Summary

Variables store temporary data inside procedures.

Control flow (IF, WHILE) lets you run code based on conditions.

Use DECLARE to create variables and SET to assign values.

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