INNER JOIN with ON condition in SQL - Step-by-Step Execution

Concept Flow - INNER JOIN with ON condition

Start with Table A

↓

Apply ON condition

↓

Match rows where ON condition is true

↓

Combine matched rows

↓

Result Table

↓

Output

We start with two tables, check each pair of rows against the ON condition, keep only matching pairs, then combine their columns into the result.

Execution Sample

SQL

SELECT A.id, A.name, B.salary
FROM Employees A
INNER JOIN Salaries B
ON A.id = B.emp_id;

This query joins Employees and Salaries tables where employee IDs match, returning employee id, name, and salary.

Execution Table

Step	Employees.id	Employees.name	Salaries.emp_id	Salaries.salary	ON Condition (A.id = B.emp_id)	Action	Output Row
1	1	Alice	1	5000	True	Include row	(1, Alice, 5000)
2	1	Alice	2	6000	False	Exclude row
3	2	Bob	1	5000	False	Exclude row
4	2	Bob	2	6000	True	Include row	(2, Bob, 6000)
5	3	Charlie	3	7000	True	Include row	(3, Charlie, 7000)
6	3	Charlie	4	8000	False	Exclude row
7	End					No more rows	Query complete

💡 All pairs checked; only rows where Employees.id equals Salaries.emp_id are included.

Variable Tracker

Variable	Start	After Step 1	After Step 4	After Step 5	Final
Current Employees.id	None	1	2	3	3
Current Salaries.emp_id	None	1	2	3	4
Output Rows	Empty	[(1, Alice, 5000)]	[(1, Alice, 5000), (2, Bob, 6000)]	[(1, Alice, 5000), (2, Bob, 6000), (3, Charlie, 7000)]	[(1, Alice, 5000), (2, Bob, 6000), (3, Charlie, 7000)]

Key Moments - 2 Insights

Why are some rows from Employees or Salaries not in the output?

Only rows where the ON condition is true (Employees.id = Salaries.emp_id) are included, as shown in execution_table rows 2, 3, and 6 where condition is false and rows are excluded.

Does INNER JOIN include rows without matches in the other table?

No, INNER JOIN only includes rows with matching pairs. The execution_table shows excluded rows when ON condition is false, confirming this.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output row at step 4?

A (1, Alice, 6000)

B (2, Bob, 5000)

C (2, Bob, 6000)

D (3, Charlie, 7000)

💡 Hint

Check the Output Row column at step 4 in the execution_table.

At which step does the ON condition first become false?

A Step 2

B Step 1

C Step 3

D Step 5

💡 Hint

Look at the ON Condition column in the execution_table to find the first 'False'.

If Salaries.emp_id 4 had a matching Employees.id 4, what would happen to the output?

A All rows would be excluded

B A new output row with id 4 would be added

C No change, because INNER JOIN ignores new matches

D Only Salaries rows would appear

💡 Hint

Refer to variable_tracker showing how output rows grow when ON condition is true.

Concept Snapshot

INNER JOIN combines rows from two tables where the ON condition is true.
Syntax: SELECT columns FROM A INNER JOIN B ON condition;
Only matching rows appear in the result.
Non-matching rows are excluded.
Useful to combine related data from two tables.

Full Transcript

This visual execution shows how INNER JOIN works by checking each pair of rows from two tables against the ON condition. Only pairs where the condition is true are included in the output. The example joins Employees and Salaries tables on employee ID. The execution table traces each step, showing which rows match and are included or excluded. The variable tracker shows how output rows build up. Key moments clarify why some rows are excluded and that INNER JOIN only keeps matching rows. The quiz tests understanding of output rows and condition checks. The snapshot summarizes the INNER JOIN concept and syntax.