INNER JOIN syntax in SQL - Step-by-Step Execution

Concept Flow - INNER JOIN syntax

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 one result row

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Output combined rows as result set

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End

INNER JOIN takes two tables, compares rows based on a condition, and outputs only rows where the condition matches.

Execution Sample

SQL

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, showing customers with their orders only if they have matching IDs.

Execution Table

Step	Action	Table A Row	Table B Row	Join Condition (A.id = B.customer_id)	Result Row Produced
1	Compare first row of A with first row of B	id=1, name='Alice'	order_id=101, customer_id=1	True	id=1, name='Alice', order_id=101
2	Compare first row of A with second row of B	id=1, name='Alice'	order_id=102, customer_id=2	False	No row
3	Compare second row of A with first row of B	id=2, name='Bob'	order_id=101, customer_id=1	False	No row
4	Compare second row of A with second row of B	id=2, name='Bob'	order_id=102, customer_id=2	True	id=2, name='Bob', order_id=102
5	Compare third row of A with first row of B	id=3, name='Carol'	order_id=101, customer_id=1	False	No row
6	Compare third row of A with second row of B	id=3, name='Carol'	order_id=102, customer_id=2	False	No row
7	No more rows to compare				End of join

💡 All rows compared; only rows with matching customer IDs included in result.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
Current Table A Row	None	id=1, name='Alice'	id=1, name='Alice'	id=2, name='Bob'	id=3, name='Carol'
Current Table B Row	None	order_id=101, customer_id=1	order_id=102, customer_id=2	order_id=102, customer_id=2	order_id=102, customer_id=2
Join Condition Result	None	True	False	True	False
Result Rows Produced	Empty	[id=1, Alice, 101]	[id=1, Alice, 101]	[id=1, Alice, 101], [id=2, Bob, 102]	[id=1, Alice, 101], [id=2, Bob, 102]

Key Moments - 2 Insights

Why are some rows from Table A not in the result?

Rows from Table A only appear if they match a row in Table B on the join condition. See execution_table rows 2, 3, 5, 6 where condition is False, so no result row is produced.

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

No. INNER JOIN only outputs rows where the join condition is True. Rows without matches are skipped, as shown in execution_table where some comparisons produce no result.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result row produced at Step 4?

A id=1, name='Alice', order_id=102

B id=2, name='Bob', order_id=102

C id=3, name='Carol', order_id=101

D No row

💡 Hint

Check the 'Result Row Produced' column at Step 4 in the execution_table.

At which step does the join condition first become False?

A Step 2

B Step 1

C Step 3

D Step 5

💡 Hint

Look at the 'Join Condition' column in execution_table rows 1-3.

If a new row in Table B had customer_id=3, how would the result change?

A No change, because INNER JOIN only matches existing rows

B All rows from Table A would appear in the result

C A new result row would appear matching Table A's id=3

D The query would return an error

💡 Hint

Refer to variable_tracker showing how matching customer_id produces result rows.

Concept Snapshot

INNER JOIN syntax:
SELECT columns
FROM TableA
INNER JOIN TableB ON TableA.key = TableB.key;

It returns rows where keys match in both tables.
Rows without matches are excluded.
Used to combine related data from two tables.

Full Transcript

INNER JOIN combines two tables by matching rows where a specified condition is true. It compares each row from the first table with each row from the second table. Only rows where the join condition matches are included in the output. Rows without a matching partner in the other table are not shown. This is useful to find related data that exists in both tables. The example query joins Customers and Orders on customer ID, showing only customers who have orders. The execution table shows step-by-step how each row pair is compared and which produce results. Variables track the current rows and condition results. Key moments clarify why some rows are excluded and how matching works. The quiz tests understanding of join results and condition evaluation. The snapshot summarizes the syntax and behavior of INNER JOIN.