Creating stored procedures in MySQL

Introduction

Stored procedures help you save a set of SQL commands to run later easily. They make repeating tasks faster and keep your work organized.

You want to run the same SQL commands many times without typing them again.

You need to perform a complex task that involves multiple SQL steps.

You want to share a common task with others without giving them the full SQL code.

You want to improve performance by reducing repeated parsing of SQL commands.

You want to keep your database logic in one place for easier updates.

Syntax

MySQL

CREATE PROCEDURE procedure_name (parameters)
BEGIN
  -- SQL statements
END

Use CREATE PROCEDURE to start making a stored procedure.

Parameters are optional and can be used to pass values into the procedure.

Examples

A simple procedure that returns a greeting message.

MySQL

CREATE PROCEDURE simpleProcedure()
BEGIN
  SELECT 'Hello, world!';
END

A procedure that takes a user ID and returns the matching user record.

MySQL

CREATE PROCEDURE getUserById(IN userId INT)
BEGIN
  SELECT * FROM users WHERE id = userId;
END

A procedure that adds two numbers and returns the result using an OUT parameter.

MySQL

CREATE PROCEDURE addNumbers(IN a INT, IN b INT, OUT sum INT)
BEGIN
  SET sum = a + b;
END

Sample Program

This creates a procedure named getProductPrice that takes a product ID and returns its price. Then it calls the procedure with product ID 2.

MySQL

DELIMITER $$
CREATE PROCEDURE getProductPrice(IN productId INT)
BEGIN
  SELECT price FROM products WHERE id = productId;
END$$
DELIMITER ;

CALL getProductPrice(2);

Important Notes

Remember to change the delimiter when creating procedures to avoid conflicts with semicolons inside the procedure.

Use CALL procedure_name(parameters); to run a stored procedure.

Stored procedures can help keep your database tasks organized and reusable.

Summary

Stored procedures save SQL commands to run later easily.

They can take input parameters and return results.

Use CREATE PROCEDURE and CALL to create and run them.

Practice

1. What is the main purpose of a stored procedure in MySQL?

easy

A. To create a new database

B. To save a set of SQL commands for reuse later

C. To delete tables automatically

D. To backup the entire database

Solution

1. Step 1: Understand stored procedure concept

   A stored procedure is a saved set of SQL commands that can be run multiple times without rewriting.

2. Step 2: Compare options with definition

   Only To save a set of SQL commands for reuse later correctly describes this purpose; others describe unrelated tasks.

3. Final Answer:

   To save a set of SQL commands for reuse later -> Option B

4. Quick Check:

   Stored procedure = saved SQL commands [OK]

Hint: Stored procedures store SQL commands to run later [OK]

Common Mistakes:

• Confusing stored procedures with database creation
• Thinking stored procedures delete tables
• Assuming stored procedures backup data

2. Which of the following is the correct syntax to create a stored procedure named GetUsers without parameters?

easy

A. CREATE PROCEDURE GetUsers() { SELECT * FROM users; }

B. CREATE PROCEDURE GetUsers BEGIN SELECT * FROM users; END;

C. CREATE PROCEDURE GetUsers() SELECT * FROM users;

D. CREATE PROCEDURE GetUsers() BEGIN SELECT * FROM users; END;

Solution

1. Step 1: Recall correct stored procedure syntax

   MySQL requires parentheses for parameters (empty if none), BEGIN and END to mark procedure body.

2. Step 2: Check each option

   CREATE PROCEDURE GetUsers() BEGIN SELECT * FROM users; END; uses parentheses and BEGIN...END correctly. Others miss parentheses, BEGIN/END, or use wrong braces.

3. Final Answer:

   CREATE PROCEDURE GetUsers() BEGIN SELECT * FROM users; END; -> Option D

4. Quick Check:

   Syntax needs () and BEGIN...END [OK]

Hint: Use () even if no parameters, and BEGIN...END block [OK]

Common Mistakes:

• Omitting parentheses after procedure name
• Missing BEGIN and END keywords
• Using curly braces instead of BEGIN...END

3. Given the stored procedure:

CREATE PROCEDURE AddNumbers(IN a INT, IN b INT, OUT sum INT)
BEGIN
  SET sum = a + b;
END;

What will be the value of sum after calling CALL AddNumbers(3, 5, @result); SELECT @result;?

medium

A. 8

B. 35

C. NULL

D. Syntax error

Solution

1. Step 1: Understand procedure parameters and call

   The procedure takes two input integers a and b, and outputs their sum into the OUT parameter sum.

2. Step 2: Analyze the call and output

   Calling with 3 and 5 sets sum = 3 + 5 = 8. The OUT parameter is stored in @result, so SELECT @result returns 8.

3. Final Answer:

   8 -> Option A

4. Quick Check:

   3 + 5 = 8 [OK]

Hint: OUT parameters store results in user variables like @result [OK]

Common Mistakes:

• Confusing concatenation with addition
• Expecting sum to be NULL before assignment
• Forgetting to use user variable for OUT parameter

4. Identify the error in this stored procedure:

CREATE PROCEDURE GetCount()
BEGIN
  SELECT COUNT(*) FROM users
END;

medium

A. Missing semicolon after SELECT statement

B. Procedure name cannot have parentheses

C. BEGIN and END keywords are not allowed

D. COUNT(*) is invalid syntax

Solution

1. Step 1: Check syntax inside procedure body

   Each statement inside BEGIN...END must end with a semicolon in MySQL.

2. Step 2: Identify missing semicolon

   The SELECT COUNT(*) FROM users statement lacks a semicolon before END; causing syntax error.

3. Final Answer:

   Missing semicolon after SELECT statement -> Option A

4. Quick Check:

   Statements inside BEGIN...END need semicolons [OK]

Hint: End each statement with semicolon inside BEGIN...END [OK]

Common Mistakes:

• Omitting semicolon after SQL statements
• Thinking parentheses are not allowed in procedure name
• Believing COUNT(*) is invalid

5. You want to create a stored procedure UpdateSalary that increases an employee's salary by a given percentage. Which of the following is the correct way to write it?

hard

A. CREATE PROCEDURE UpdateSalary(IN empId INT, IN pct INT) UPDATE employees SET salary = salary * pct WHERE id = empId;

B. CREATE PROCEDURE UpdateSalary(empId INT, pct DECIMAL(5,2)) BEGIN UPDATE employees SET salary = salary + pct WHERE id = empId; END;

C. CREATE PROCEDURE UpdateSalary(IN empId INT, IN pct DECIMAL(5,2)) BEGIN UPDATE employees SET salary = salary * (1 + pct/100) WHERE id = empId; END;

D. CREATE PROCEDURE UpdateSalary(IN empId INT, IN pct DECIMAL(5,2)) BEGIN UPDATE employees SET salary = salary + pct WHERE id = empId END

Solution

1. Step 1: Check parameter declarations and body structure

   CREATE PROCEDURE UpdateSalary(IN empId INT, IN pct DECIMAL(5,2)) BEGIN UPDATE employees SET salary = salary * (1 + pct/100) WHERE id = empId; END; correctly declares input parameters with IN and uses BEGIN...END block.

2. Step 2: Verify salary update logic and syntax

   CREATE PROCEDURE UpdateSalary(IN empId INT, IN pct DECIMAL(5,2)) BEGIN UPDATE employees SET salary = salary * (1 + pct/100) WHERE id = empId; END; updates salary by multiplying with (1 + pct/100), which correctly increases salary by percentage. It also ends statements with semicolon.

3. Step 3: Identify errors in other options

   CREATE PROCEDURE UpdateSalary(empId INT, pct DECIMAL(5,2)) BEGIN UPDATE employees SET salary = salary + pct WHERE id = empId; END; lacks IN keywords and adds pct directly (not percentage). CREATE PROCEDURE UpdateSalary(IN empId INT, IN pct INT) UPDATE employees SET salary = salary * pct WHERE id = empId; misses BEGIN...END and uses wrong pct type. CREATE PROCEDURE UpdateSalary(IN empId INT, IN pct DECIMAL(5,2)) BEGIN UPDATE employees SET salary = salary + pct WHERE id = empId END misses semicolon before END.

4. Final Answer:

   CREATE PROCEDURE UpdateSalary(IN empId INT, IN pct DECIMAL(5,2)) BEGIN UPDATE employees SET salary = salary * (1 + pct/100) WHERE id = empId; END; -> Option C

5. Quick Check:

   Use IN params, BEGIN...END, and correct percentage formula [OK]

Hint: Use IN params and multiply salary by (1 + pct/100) [OK]

Common Mistakes:

• Forgetting IN keyword for parameters
• Adding percentage directly instead of multiplying
• Missing BEGIN...END or semicolons