INNER JOIN execution in PostgreSQL - Step-by-Step Execution

Concept Flow - INNER JOIN execution

Start with Table A

↓

Pick one row from Table A

↓

Find matching rows in Table B where join condition is true

↓

Combine Table A row with each matching Table B row

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Add combined rows to result set

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Repeat for next row in Table A

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No more rows in Table A?

Yes→Return result set

↩Back to pick next row

INNER JOIN takes each row from the first table and finds matching rows in the second table based on the join condition, combining them into the result.

Execution Sample

PostgreSQL

SELECT A.id, A.name, B.order_id
FROM Customers A
INNER JOIN Orders B ON A.id = B.customer_id;

This query joins Customers and Orders tables, returning customers with their matching orders.

Execution Table

Step	Current Row from Customers (A)	Matching Rows from Orders (B)	Action	Result Rows Added
1	A.id=1, A.name='Alice'	B.order_id=101, B.customer_id=1 B.order_id=102, B.customer_id=1	Match found for 2 orders	(1, 'Alice', 101), (1, 'Alice', 102)
2	A.id=2, A.name='Bob'	B.order_id=103, B.customer_id=2	Match found for 1 order	(2, 'Bob', 103)
3	A.id=3, A.name='Charlie'	No matching orders	No match, no rows added	None
4	No more rows in Customers		Stop execution	Final result returned

💡 All rows in Customers processed; INNER JOIN returns only matched rows.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
Current Customer Row	None	id=1, name='Alice'	id=2, name='Bob'	id=3, name='Charlie'	None
Matching Orders	None	order_id=101,102	order_id=103	None	None
Result Rows	Empty	2 rows added	1 row added	No rows added	3 rows total

Key Moments - 2 Insights

Why does the row for 'Charlie' not appear in the result?

Because there are no matching rows in Orders for Charlie's id (3), INNER JOIN excludes rows without matches as shown in execution_table row 3.

How does INNER JOIN handle multiple matches for one row?

It creates one result row for each matching row in the second table, as seen in execution_table row 1 where Alice matches two orders.

Visual Quiz - 3 Questions

Look at the execution table, how many result rows are added after processing Bob's row?

A 0

B 1

C 2

D 3

💡 Hint

Check execution_table row 2 under 'Result Rows Added'

At which step does the INNER JOIN stop processing rows?

A Step 2

B Step 3

C Step 4

D Step 1

💡 Hint

Look at execution_table row 4 where it says 'Stop execution'

If the Orders table had an extra order for customer_id=3, how would the result change at step 3?

A One row added at step 3

B No rows added at step 3

C Multiple rows added at step 3

D Step 3 would be skipped

💡 Hint

Refer to variable_tracker 'Matching Orders' and execution_table row 3 for no matches currently

Concept Snapshot

INNER JOIN combines rows from two tables where the join condition matches.
Only rows with matches in both tables appear in the result.
Each matching pair creates one combined row.
No match means the row is excluded.
Syntax: SELECT columns FROM A INNER JOIN B ON condition;

Full Transcript

INNER JOIN works by taking each row from the first table and searching for matching rows in the second table based on the join condition. For each match found, it combines the rows into one result row. If no match is found, that row from the first table is not included in the output. This process repeats for all rows in the first table until none remain. The example query joins Customers and Orders on customer ID, showing how multiple orders for one customer create multiple result rows, and customers without orders are excluded.