INNER JOIN execution in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does an INNER JOIN do in SQL?

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

beginner

In an INNER JOIN, what happens if a row in one table has no matching row in the other table?

That row is not included in the result because INNER JOIN only shows rows with matches in both tables.

intermediate

How does PostgreSQL execute an INNER JOIN internally?

PostgreSQL finds pairs of rows from both tables that satisfy the join condition, often using indexes or scanning tables, then combines those rows into the result.

intermediate

What is the difference between INNER JOIN and LEFT JOIN in terms of execution?

INNER JOIN returns only matching rows from both tables, while LEFT JOIN returns all rows from the left table and matches from the right, filling with NULLs if no match.

intermediate

Why is it important to have indexes on join columns for INNER JOIN execution?

Indexes speed up finding matching rows between tables, making INNER JOIN queries faster and more efficient.

What rows does an INNER JOIN return?

A Only rows with matching values in both tables

B All rows from the left table

C All rows from the right table

D All rows from both tables

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

If a row in the left table has no matching row in the right table, what does INNER JOIN do?

A Excludes the row from the result

B Duplicates the row

C Includes the row with NULLs for right table columns

D Returns an error

INNER JOIN excludes rows without matches in the other table.

Which of these helps PostgreSQL speed up INNER JOIN execution?

A Adding more columns to the join condition

B Using SELECT *

C Indexes on join columns

D Using ORDER BY on unrelated columns

Indexes on join columns help PostgreSQL quickly find matching rows.

What is the main difference in output between INNER JOIN and LEFT JOIN?

A INNER JOIN returns unmatched rows, LEFT JOIN does not

B LEFT JOIN returns unmatched rows from the left table, INNER JOIN does not

C INNER JOIN returns all rows, LEFT JOIN returns only matches

D They return the same rows

LEFT JOIN includes all rows from the left table, even if no match exists.

During INNER JOIN execution, what does PostgreSQL do?

A Deletes unmatched rows from tables

B Combines every row from both tables regardless of condition

C Returns only rows from the first table

D Finds matching rows based on join condition and combines them

PostgreSQL finds matching rows that satisfy the join condition and returns combined rows.

Explain how INNER JOIN works when executed in PostgreSQL.

Think about how two lists are matched by a common key.

Describe the difference between INNER JOIN and LEFT JOIN in terms of their output.

Consider what happens when one table has rows without matches in the other.

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