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

↓

Add combined rows to result set

↓

Repeat for next row in Table A

↓

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.

Practice

1. What does an INNER JOIN do in PostgreSQL?

easy

A. Combines rows from two tables where matching column values exist in both tables.

B. Returns all rows from the first table, and matching rows from the second table.

C. Returns all rows from both tables, matching or not.

D. Deletes rows from both tables where values match.

Solution

1. Step 1: Understand INNER JOIN purpose

   INNER JOIN returns rows only when there is a match in both tables based on the join condition.

2. Step 2: Compare options with INNER JOIN behavior

   Combines rows from two tables where matching column values exist in both tables. correctly describes this behavior. Options B and C describe LEFT JOIN and FULL OUTER JOIN respectively, and D is unrelated.

3. Final Answer:

   Combines rows from two tables where matching column values exist in both tables. -> Option A

4. Quick Check:

   INNER JOIN = matched rows only [OK]

Hint: INNER JOIN returns only rows with matches in both tables [OK]

Common Mistakes:

• Confusing INNER JOIN with LEFT JOIN or FULL JOIN
• Thinking INNER JOIN returns unmatched rows
• Assuming INNER JOIN deletes data

2. Which of the following is the correct syntax for an INNER JOIN in PostgreSQL?

easy

A. SELECT * FROM table1 INNER JOIN table2 ON table1.id = table2.id;

B. SELECT * FROM table1 INNER JOIN table2 WHERE table1.id = table2.id;

C. SELECT * FROM table1 INNER JOIN table2 USING (id);

D. SELECT * FROM table1 JOIN table2 ON table1.id == table2.id;

Solution

1. Step 1: Review INNER JOIN syntax

   The correct syntax uses INNER JOIN with ON clause specifying the join condition using a single equals sign (=).

2. Step 2: Check each option

   SELECT * FROM table1 INNER JOIN table2 ON table1.id = table2.id; uses correct INNER JOIN syntax with ON and =. SELECT * FROM table1 INNER JOIN table2 WHERE table1.id = table2.id; missing ON or USING clause, syntax error. SELECT * FROM table1 INNER JOIN table2 USING (id); uses correct USING syntax with parentheses. SELECT * FROM table1 JOIN table2 ON table1.id == table2.id; uses double equals (==) which is invalid in SQL.

3. Final Answer:

   SELECT * FROM table1 INNER JOIN table2 ON table1.id = table2.id; -> Option A

4. Quick Check:

   INNER JOIN uses ON with = [OK]

Hint: Use ON with single = for INNER JOIN conditions [OK]

Common Mistakes:

• Using WHERE instead of ON for join condition
• Using double equals (==) instead of single =
• Confusing USING clause applicability

3. Given the tables:
employees(id, name, dept_id)
departments(id, dept_name)
What will this query return?

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

medium

A. All employees, even if they have no matching department.

B. An error because of alias usage.

C. All departments with employee names, even if no employees exist.

D. All employees with their department names, only if dept_id matches a department id.

Solution

1. Step 1: Understand INNER JOIN effect on employees and departments

   The INNER JOIN returns rows where employees.dept_id matches departments.id, so only employees with a valid department appear.

2. Step 2: Analyze query output

   The query selects employee names and their department names for matching rows only. Employees without a matching department are excluded.

3. Final Answer:

   All employees with their department names, only if dept_id matches a department id. -> Option D

4. Quick Check:

   INNER JOIN filters unmatched rows [OK]

Hint: INNER JOIN shows only matching rows from both tables [OK]

Common Mistakes:

• Assuming unmatched employees appear
• Confusing aliases causing errors
• Thinking departments without employees appear

4. Identify the error in this INNER JOIN query:

SELECT * FROM orders INNER JOIN customers ON orders.customer_id = customers.id WHERE orders.date > '2023-01-01'

medium

A. Missing table alias causing ambiguity.

B. WHERE clause cannot be used with INNER JOIN.

C. No error; query is valid and will run correctly.

D. Incorrect join condition syntax; should use USING instead of ON.

Solution

1. Step 1: Check JOIN syntax and WHERE clause

   The INNER JOIN uses ON with a valid condition. The WHERE clause filters rows after the join, which is allowed.

2. Step 2: Verify if any syntax or semantic errors exist

   There is no ambiguity or syntax error. Aliases are optional. USING is an alternative but not required.

3. Final Answer:

   No error; query is valid and will run correctly. -> Option C

4. Quick Check:

   INNER JOIN with ON and WHERE is valid [OK]

Hint: WHERE filters after JOIN; no syntax error here [OK]

Common Mistakes:

• Thinking WHERE cannot be used with JOIN
• Confusing USING and ON as mandatory
• Assuming aliases are always required

5. You have two tables:
products(id, name)
sales(product_id, quantity)
Write a query to find product names with total quantity sold, but only for products that have sales.
Which query correctly uses INNER JOIN to achieve this?

hard

A. SELECT p.name, SUM(s.quantity) FROM sales s LEFT JOIN products p ON s.product_id = p.id GROUP BY p.name;

B. SELECT p.name, SUM(s.quantity) FROM products p INNER JOIN sales s ON p.id = s.product_id GROUP BY p.name;

C. SELECT p.name, SUM(s.quantity) FROM products p LEFT JOIN sales s ON p.id = s.product_id GROUP BY p.name;

D. SELECT p.name, SUM(s.quantity) FROM products p INNER JOIN sales s ON p.id = s.product_id WHERE p.name > 0 GROUP BY p.name;

Solution

1. Step 1: Understand requirement for products with sales only

   INNER JOIN returns only products that have matching sales records, excluding products without sales.

2. Step 2: Analyze each query

   SELECT p.name, SUM(s.quantity) FROM sales s LEFT JOIN products p ON s.product_id = p.id GROUP BY p.name; uses LEFT JOIN, includes unmatched sales (NULL p.name). SELECT p.name, SUM(s.quantity) FROM products p LEFT JOIN sales s ON p.id = s.product_id GROUP BY p.name; uses LEFT JOIN, includes products without sales. SELECT p.name, SUM(s.quantity) FROM products p INNER JOIN sales s ON p.id = s.product_id GROUP BY p.name; uses INNER JOIN correctly. SELECT p.name, SUM(s.quantity) FROM products p INNER JOIN sales s ON p.id = s.product_id WHERE p.name > 0 GROUP BY p.name; WHERE p.name > 0 causes type error.

3. Step 3: Choose best answer matching INNER JOIN usage

   SELECT p.name, SUM(s.quantity) FROM products p INNER JOIN sales s ON p.id = s.product_id GROUP BY p.name; is correct.

4. Final Answer:

   SELECT p.name, SUM(s.quantity) FROM products p INNER JOIN sales s ON p.id = s.product_id GROUP BY p.name; -> Option B

5. Quick Check:

   INNER JOIN + GROUP BY sums sales per product [OK]

Hint: Use INNER JOIN and GROUP BY to sum only matching sales [OK]

Common Mistakes:

• Using LEFT JOIN includes products without sales
• Forgetting GROUP BY causes errors
• Confusing table order in JOIN