Procedure parameters (IN, OUT, INOUT) in MySQL

Introduction

Procedure parameters let you send data into a procedure and get data back from it. They help procedures work with different values each time.

You want to give a procedure some input values to work with.

You want a procedure to return a value after running.

You want a procedure to both receive input and send back output.

You want to reuse a procedure with different data without changing its code.

You want to organize your database logic into small, manageable parts.

Syntax

MySQL

CREATE PROCEDURE procedure_name(
  IN param1 datatype,
  OUT param2 datatype,
  INOUT param3 datatype
)
BEGIN
  -- procedure body
END;

IN parameters send data into the procedure.

OUT parameters send data back from the procedure.

INOUT parameters do both: send data in and send updated data out.

Examples

This procedure takes a name as input and returns a greeting message.

MySQL

CREATE PROCEDURE greet_user(IN username VARCHAR(50))
BEGIN
  SELECT CONCAT('Hello, ', username) AS greeting;
END;

This procedure calculates total sales and sends the result back using an OUT parameter.

MySQL

CREATE PROCEDURE get_total_sales(OUT total DECIMAL(10,2))
BEGIN
  SELECT SUM(amount) INTO total FROM sales;
END;

This procedure updates the balance by adding a change amount and returns the new balance.

MySQL

CREATE PROCEDURE adjust_balance(INOUT balance DECIMAL(10,2), IN change DECIMAL(10,2))
BEGIN
  SET balance = balance + change;
END;

Sample Program

This procedure takes a current balance and a deposit amount. It adds the deposit to the balance and returns the updated balance both as an INOUT and an OUT parameter.

MySQL

DELIMITER $$
CREATE PROCEDURE update_and_get_balance(
  INOUT current_balance DECIMAL(10,2),
  IN deposit DECIMAL(10,2),
  OUT new_balance DECIMAL(10,2)
)
BEGIN
  SET current_balance = current_balance + deposit;
  SET new_balance = current_balance;
END$$
DELIMITER ;

-- Declare variables to test the procedure
SET @balance = 100.00;
SET @deposit = 50.00;
CALL update_and_get_balance(@balance, @deposit, @new_balance);
SELECT @balance AS updated_balance, @new_balance AS output_balance;

Important Notes

Use IN when you only need to send data into the procedure.

Use OUT when you want the procedure to send data back to you.

Use INOUT when you want to send data in and get the changed data back.

Summary

Procedure parameters control how data moves in and out of procedures.

IN is for input, OUT is for output, and INOUT is for both.

They help make procedures flexible and reusable with different data.

Practice

1. What does the IN parameter type in a MySQL stored procedure do?

easy

A. It declares a variable inside the procedure without passing values.

B. It passes a value into the procedure but does not allow changes to be sent back.

C. It allows both passing a value in and sending a changed value back.

D. It allows the procedure to send a value back to the caller only.

Solution

1. Step 1: Understand the role of IN parameters

   IN parameters are used to pass values into a procedure. They act like input-only variables.

2. Step 2: Clarify output behavior of IN parameters

   Changes to IN parameters inside the procedure do not affect the caller's variables; they are input-only.

3. Final Answer:

   It passes a value into the procedure but does not allow changes to be sent back. -> Option B

4. Quick Check:

   IN = input only [OK]

Hint: IN means input only, no output changes sent back [OK]

Common Mistakes:

• Thinking IN parameters can send data back
• Confusing IN with OUT or INOUT
• Assuming IN parameters can be modified outside

2. Which of the following is the correct syntax to declare an OUT parameter in a MySQL stored procedure?

easy

A. CREATE PROCEDURE myProc(param OUT INT) BEGIN END;

B. CREATE PROCEDURE myProc(INOUT param INT) BEGIN END;

C. CREATE PROCEDURE myProc(OUT param INT) BEGIN END;

D. CREATE PROCEDURE myProc(param INT OUT) BEGIN END;

Solution

1. Step 1: Recall correct parameter declaration order

   In MySQL, the parameter mode (IN, OUT, INOUT) comes before the parameter name and type.

2. Step 2: Check each option's syntax

   CREATE PROCEDURE myProc(OUT param INT) BEGIN END; uses 'OUT param INT' which is correct. Placing OUT after the parameter name or type is invalid syntax.

3. Final Answer:

   CREATE PROCEDURE myProc(OUT param INT) BEGIN END; -> Option C

4. Quick Check:

   Parameter mode before name and type [OK]

Hint: Parameter mode (IN/OUT/INOUT) always comes first [OK]

Common Mistakes:

• Placing OUT after parameter name or type
• Mixing parameter mode with data type
• Omitting BEGIN and END block

3. Consider this procedure call and definition:

CREATE PROCEDURE addValues(IN a INT, INOUT b INT, OUT c INT)
BEGIN
  SET b = b + a;
  SET c = a + b;
END;

CALL addValues(5, @varB, @varC);

If @varB is initially 10, what will be the values of @varB and @varC after the call?

medium

A. @varB = 15, @varC = 20

B. @varB = 10, @varC = 15

C. @varB = 5, @varC = 10

D. @varB = 15, @varC = 10

Solution

1. Step 1: Understand parameter roles and initial values

   a = 5 (IN), b = 10 (INOUT), c is OUT (initially NULL). INOUT means b can be read and updated.

2. Step 2: Calculate new values inside procedure

   SET b = b + a = 10 + 5 = 15; SET c = a + b = 5 + 15 = 20.

3. Final Answer:

   @varB = 15, @varC = 20 -> Option A

4. Quick Check:

   INOUT b updated, OUT c set correctly [OK]

Hint: INOUT changes persist; OUT must be set inside procedure [OK]

Common Mistakes:

• Assuming INOUT parameter does not change
• Confusing OUT parameter initial value
• Ignoring that OUT must be assigned inside procedure

4. Given this procedure:

CREATE PROCEDURE testProc(INOUT x INT)
BEGIN
  SET x = x + 1;
END;

Which of the following calls will cause an error?

medium

A. CALL testProc(@varX); -- @varX is a session variable

B. CALL testProc(NULL);

C. SET @varX = 10; CALL testProc(@varX);

D. CALL testProc(5);

Solution

1. Step 1: Understand INOUT parameter requirements

   INOUT parameters require a variable to pass by reference, so the procedure can read and modify it.

2. Step 2: Analyze each call

   CALL testProc(5); passes a literal (5), which is not a variable, causing an error. The other options pass variables or NULL variables, which are allowed.

3. Final Answer:

   CALL testProc(5); -> Option D

4. Quick Check:

   INOUT needs variable, not literal [OK]

Hint: INOUT requires variable, not constant literal [OK]

Common Mistakes:

• Passing literals to INOUT parameters
• Confusing INOUT with IN parameters
• Assuming NULL literal is invalid for INOUT

5. You want to create a procedure that takes two numbers, adds them, and returns the sum and the product. Which parameter types should you use for the inputs and outputs?

hard

A. IN for both inputs, OUT for sum and product outputs

B. INOUT for inputs, IN for outputs

C. OUT for inputs, IN for outputs

D. IN for inputs, INOUT for outputs

Solution

1. Step 1: Identify input and output roles

   The two numbers are inputs only, so they should be IN parameters. The sum and product are results to send back, so they should be OUT parameters.

2. Step 2: Match parameter types to roles

   IN parameters accept input values; OUT parameters return output values. INOUT is for parameters used as both input and output, which is not needed here.

3. Final Answer:

   IN for both inputs, OUT for sum and product outputs -> Option A

4. Quick Check:

   Inputs = IN, Outputs = OUT [OK]

Hint: Use IN for inputs, OUT for outputs to separate roles clearly [OK]

Common Mistakes:

• Using INOUT unnecessarily for outputs
• Confusing OUT as input
• Using IN for output parameters