INNER JOIN execution in PostgreSQL

Introduction

INNER JOIN helps combine rows from two tables when they have matching values in a related column.

You want to see orders along with customer details where the order exists.

You need to find employees and their departments only if the employee is assigned to a department.

You want to list products and their suppliers but only for products that have suppliers.

You want to combine student records with their enrolled courses only if enrollment exists.

Syntax

PostgreSQL

SELECT columns
FROM table1
INNER JOIN table2
ON table1.common_column = table2.common_column;

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

If no match is found, the row is not included in the result.

Examples

This gets employee names with their department names only if the employee belongs to a department.

PostgreSQL

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

This lists order IDs with customer names only for orders that have a matching customer.

PostgreSQL

SELECT orders.id, customers.name
FROM orders
INNER JOIN customers
ON orders.customer_id = customers.id;

Sample Program

This example creates two tables, inserts data, and uses INNER JOIN to show orders with matching customers. The order with customer_id 3 is excluded because no such customer exists.

PostgreSQL

CREATE TABLE customers (
  id SERIAL PRIMARY KEY,
  name VARCHAR(50)
);

CREATE TABLE orders (
  id SERIAL PRIMARY KEY,
  customer_id INT,
  product VARCHAR(50)
);

INSERT INTO customers (name) VALUES ('Alice'), ('Bob');
INSERT INTO orders (customer_id, product) VALUES (1, 'Book'), (2, 'Pen'), (3, 'Notebook');

SELECT orders.id AS order_id, customers.name AS customer_name, orders.product
FROM orders
INNER JOIN customers
ON orders.customer_id = customers.id
ORDER BY orders.id;

Important Notes

INNER JOIN only shows rows where both tables have matching values in the join column.

If you want to keep all rows from one table even if no match exists, consider LEFT JOIN instead.

Always specify the join condition with ON to avoid a large, unintended result.

Summary

INNER JOIN combines rows from two tables based on matching column values.

Only rows with matches in both tables appear in the result.

Use INNER JOIN when you want related data from both tables and no unmatched rows.

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