Procedure parameters (IN, OUT, INOUT) in MySQL - Step-by-Step Execution

Concept Flow - Procedure parameters (IN, OUT, INOUT)

Start Procedure Call

↓

Pass IN parameter value

↓

Procedure executes using IN value

↓

Modify OUT and INOUT parameters inside procedure

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Return OUT and INOUT values to caller

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End Procedure Call

This flow shows how IN parameters are passed in, OUT and INOUT parameters are modified inside the procedure, and then returned to the caller.

Execution Sample

MySQL

CREATE PROCEDURE example_proc(IN in_val INT, OUT out_val INT, INOUT inout_val INT)
BEGIN
  SET out_val = in_val + 10;
  SET inout_val = inout_val + in_val;
END

This procedure takes an IN parameter, modifies OUT and INOUT parameters based on it.

Execution Table

Step	Parameter	Initial Value	Action	Value After Action
1	in_val	5 (passed in)	Use as input	5
2	out_val	NULL	Set to in_val + 10	15
3	inout_val	3 (passed in)	Add in_val to inout_val	8
4	Procedure ends	-	Return out_val and inout_val to caller	-

💡 Procedure ends after setting OUT and INOUT parameters; values returned to caller.

Variable Tracker

Parameter	Start	After Step 2	After Step 3	Final
in_val	5	5	5	5
out_val	NULL	15	15	15
inout_val	3	3	8	8

Key Moments - 3 Insights

Why can't we change the IN parameter inside the procedure?

IN parameters are read-only inside the procedure, so their value stays the same as shown in execution_table step 1.

How do OUT parameters get their value back to the caller?

OUT parameters are assigned inside the procedure (step 2) and their final value is returned after the procedure ends (step 4).

What happens to INOUT parameters during execution?

INOUT parameters start with a value from the caller (step 1), can be read and changed inside the procedure (step 3), and the updated value is returned (step 4).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of out_val after step 2?

A 15

B 5

C NULL

D 8

💡 Hint

Check the 'Value After Action' column for out_val at step 2 in the execution_table.

At which step does inout_val change its value?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Action' column for inout_val in the execution_table to see when it is modified.

If the initial in_val was 7 instead of 5, what would out_val be after step 2?

A 7

B 12

C 17

D NULL

💡 Hint

out_val is set to in_val + 10 at step 2; check execution_table logic.

Concept Snapshot

Procedure parameters in MySQL:
- IN: input only, cannot be changed inside procedure.
- OUT: output only, set inside procedure and returned.
- INOUT: input and output, can be read and changed.
Use SET inside procedure to assign OUT and INOUT values.
Caller receives OUT and INOUT updated values after execution.

Full Transcript

This visual execution shows how MySQL procedure parameters work. The IN parameter is passed in and used as input without change. The OUT parameter starts null and is assigned a new value inside the procedure. The INOUT parameter starts with a value, is modified inside, and the new value is returned. The execution table tracks each step and parameter value changes. Key moments clarify common confusions about parameter roles. The quiz tests understanding of parameter values at different steps. The snapshot summarizes the behavior of IN, OUT, and INOUT parameters in procedures.