INNER JOIN in MySQL - Step-by-Step Execution

Concept Flow - INNER JOIN

Start with Table A

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Start with Table B

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Compare rows from A and B

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Match rows where join condition is true

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Combine matched rows into result

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Return combined rows only

INNER JOIN takes two tables and returns only the rows where the join condition matches in both tables.

Execution Sample

MySQL

SELECT employees.name, departments.dept_name
FROM employees
INNER JOIN departments
ON employees.dept_id = departments.id;

This query returns employee names with their department names, only where the department ID matches in both tables.

Execution Table

Step	employees row	departments row	Join Condition (employees.dept_id = departments.id)	Action	Output row
1	{id:1, name:'Alice', dept_id:10}	{id:10, dept_name:'HR'}	10 = 10 (True)	Combine rows	{name:'Alice', dept_name:'HR'}
2	{id:1, name:'Alice', dept_id:10}	{id:20, dept_name:'Sales'}	10 = 20 (False)	Skip	No output
3	{id:2, name:'Bob', dept_id:20}	{id:10, dept_name:'HR'}	20 = 10 (False)	Skip	No output
4	{id:2, name:'Bob', dept_id:20}	{id:20, dept_name:'Sales'}	20 = 20 (True)	Combine rows	{name:'Bob', dept_name:'Sales'}
5	{id:3, name:'Carol', dept_id:30}	{id:10, dept_name:'HR'}	30 = 10 (False)	Skip	No output
6	{id:3, name:'Carol', dept_id:30}	{id:20, dept_name:'Sales'}	30 = 20 (False)	Skip	No output
7	No more employees rows	N/A	N/A	End	Final output rows returned

💡 All rows from employees checked against all departments; only matching dept_id rows combined and returned.

Variable Tracker

Variable	Start	After Step 1	After Step 4	After Step 7 (End)
Current employees row	None	{id:1, name:'Alice', dept_id:10}	{id:2, name:'Bob', dept_id:20}	None
Current departments row	None	{id:10, dept_name:'HR'}	{id:20, dept_name:'Sales'}	None
Output rows	[]	[{name:'Alice', dept_name:'HR'}]	[{name:'Alice', dept_name:'HR'}, {name:'Bob', dept_name:'Sales'}]	[{name:'Alice', dept_name:'HR'}, {name:'Bob', dept_name:'Sales'}]

Key Moments - 2 Insights

Why are some rows from employees not in the output?

Rows from employees without a matching dept_id in departments are skipped, as shown in steps 5 and 6 where Carol's dept_id 30 does not match any department id.

Does INNER JOIN include rows with no matching partner in the other table?

No, INNER JOIN only returns rows where the join condition is true for both tables, as seen in the execution table where only matching dept_id rows produce output.

Visual Quiz - 3 Questions

Look at the execution table, what is the output row at Step 4?

A {name:'Alice', dept_name:'HR'}

B {name:'Bob', dept_name:'Sales'}

C No output

D {name:'Carol', dept_name:'HR'}

💡 Hint

Check the 'Output row' column at Step 4 in the execution_table.

At which step does the join condition first become false?

A Step 1

B Step 4

C Step 2

D Step 7

💡 Hint

Look at the 'Join Condition' column in the execution_table for the first 'False' value.

If a new department with id 30 is added, how would the output change?

A Carol's row would appear in the output

B No change in output

C All employees would be excluded

D Only departments with id 10 and 20 would appear

💡 Hint

Refer to the variable_tracker and execution_table where Carol's dept_id 30 had no match before.

Concept Snapshot

INNER JOIN syntax:
SELECT columns
FROM table1
INNER JOIN table2
ON table1.column = table2.column;

Returns rows only where join condition matches in both tables.
Rows without matches are excluded.

Full Transcript

INNER JOIN combines rows from two tables where the join condition is true. It compares each row from the first table with each row from the second table. Only rows with matching values in the join columns are combined and returned. Rows without a match in the other table are skipped. For example, employees are joined with departments on dept_id. Only employees with a matching department id appear in the result. This is shown step-by-step in the execution table, where each employee row is checked against department rows, and only matching pairs produce output rows.