LEFT JOIN vs RIGHT JOIN decision in SQL - Quick Revision & Key Differences

Recall & Review

beginner

What does a LEFT JOIN do in SQL?

A LEFT JOIN returns all rows from the left table and the matching rows from the right table. If there is no match, the result is NULL on the right side.

beginner

What does a RIGHT JOIN do in SQL?

A RIGHT JOIN returns all rows from the right table and the matching rows from the left table. If there is no match, the result is NULL on the left side.

intermediate

When should you use LEFT JOIN instead of RIGHT JOIN?

Use LEFT JOIN when you want to keep all rows from the first (left) table and add matching data from the second (right) table. It is often easier to read and understand.

intermediate

Can a RIGHT JOIN be rewritten as a LEFT JOIN?

Yes. A RIGHT JOIN can be rewritten as a LEFT JOIN by swapping the order of the tables. This helps keep queries consistent and easier to read.

beginner

Why might developers prefer LEFT JOIN over RIGHT JOIN?

Developers often prefer LEFT JOIN because it reads naturally from left to right, matching the order of tables in the query. This improves clarity and reduces confusion.

What does a LEFT JOIN return?

A All rows from both tables

B All rows from the left table and matching rows from the right table

C Only matching rows from both tables

D All rows from the right table and matching rows from the left table

LEFT JOIN returns all rows from the left table and matching rows from the right table, filling with NULLs if no match.

How can you rewrite a RIGHT JOIN as a LEFT JOIN?

A Swap the order of the tables and use LEFT JOIN

B Change RIGHT JOIN to INNER JOIN

C Use FULL JOIN instead

D You cannot rewrite RIGHT JOIN as LEFT JOIN

RIGHT JOIN can be rewritten as LEFT JOIN by swapping the tables' order.

Which join is usually easier to read and understand?

A LEFT JOIN

B RIGHT JOIN

C FULL JOIN

D CROSS JOIN

LEFT JOIN is often easier to read because it follows the natural left-to-right order.

If you want all rows from the right table regardless of matches, which join do you use?

A CROSS JOIN

B LEFT JOIN

C INNER JOIN

D RIGHT JOIN

RIGHT JOIN returns all rows from the right table and matching rows from the left.

What happens to unmatched rows in the right table when using LEFT JOIN?

A They are included with NULLs for left table columns

B They are included with NULLs for right table columns

C They are excluded

D They cause an error

LEFT JOIN excludes unmatched rows from the right table; only unmatched left table rows appear with NULLs on the right.

Explain the difference between LEFT JOIN and RIGHT JOIN in simple terms.

Think about which table's rows you want to keep fully.

Describe when and why you might choose LEFT JOIN over RIGHT JOIN in a query.

Consider readability and table order.

Practice

1. Which JOIN type keeps all rows from the first (left) table, even if there is no matching row in the second (right) table?

easy

A. RIGHT JOIN

B. INNER JOIN

C. LEFT JOIN

D. FULL JOIN

Solution

1. Step 1: Understand LEFT JOIN behavior

   LEFT JOIN returns all rows from the first table and matching rows from the second table. If no match, NULLs appear for second table columns.

2. Step 2: Compare with RIGHT JOIN

   RIGHT JOIN keeps all rows from the second table, not the first. INNER JOIN only returns matching rows. FULL JOIN returns all rows from both tables.

3. Final Answer:

   LEFT JOIN -> Option C

4. Quick Check:

   LEFT JOIN keeps all left table rows [OK]

Hint: LEFT JOIN keeps all from first table, RIGHT JOIN from second [OK]

Common Mistakes:

• Confusing LEFT JOIN with RIGHT JOIN
• Thinking INNER JOIN keeps unmatched rows
• Assuming FULL JOIN keeps only one table's rows

2. Which of the following is the correct syntax to perform a RIGHT JOIN between tables Employees and Departments on Employees.DeptID = Departments.ID?

easy

A. SELECT * FROM Employees RIGHT JOIN Departments ON Employees.DeptID = Departments.ID;

B. SELECT * FROM Employees LEFT JOIN Departments ON Employees.DeptID = Departments.ID;

C. SELECT * FROM Employees RIGHT JOIN Departments WHERE Employees.DeptID = Departments.ID;

D. SELECT * FROM Employees JOIN Departments ON Employees.DeptID = Departments.ID;

Solution

1. Step 1: Check JOIN syntax correctness

   RIGHT JOIN requires ON clause to specify join condition. SELECT * FROM Employees RIGHT JOIN Departments ON Employees.DeptID = Departments.ID; uses correct syntax with ON clause.

2. Step 2: Identify errors in other options

   SELECT * FROM Employees LEFT JOIN Departments ON Employees.DeptID = Departments.ID; uses LEFT JOIN, not RIGHT JOIN. SELECT * FROM Employees RIGHT JOIN Departments WHERE Employees.DeptID = Departments.ID; uses WHERE instead of ON, which is incorrect for JOIN condition. SELECT * FROM Employees JOIN Departments ON Employees.DeptID = Departments.ID; uses JOIN without specifying LEFT or RIGHT, defaults to INNER JOIN.

3. Final Answer:

   SELECT * FROM Employees RIGHT JOIN Departments ON Employees.DeptID = Departments.ID; -> Option A

4. Quick Check:

   RIGHT JOIN needs ON clause [OK]

Hint: RIGHT JOIN syntax requires ON, not WHERE [OK]

Common Mistakes:

• Using WHERE instead of ON for join condition
• Confusing LEFT JOIN with RIGHT JOIN syntax
• Omitting JOIN type defaults to INNER JOIN

3. Given tables Authors and Books, what will be the result of this query?

SELECT Authors.Name, Books.Title FROM Authors LEFT JOIN Books ON Authors.ID = Books.AuthorID;

Assuming Authors has 3 rows and Books has 2 rows, with one author having no books, what will the output include?

medium

A. Only authors who have books, excluding authors without books

B. All 3 authors with their book titles; NULL for authors without books

C. All books with their authors; NULL for books without authors

D. All authors and all books, matched and unmatched

Solution

1. Step 1: Understand LEFT JOIN output

   LEFT JOIN keeps all rows from Authors (left table). For authors without matching books, Books.Title will be NULL.

2. Step 2: Analyze given data

   Authors has 3 rows, Books has 2 rows. One author has no books, so that author appears with NULL for book title.

3. Final Answer:

   All 3 authors with their book titles; NULL for authors without books -> Option B

4. Quick Check:

   LEFT JOIN keeps all left rows, fills NULLs for no match [OK]

Hint: LEFT JOIN keeps all left rows, unmatched right columns are NULL [OK]

Common Mistakes:

• Thinking unmatched left rows are excluded
• Confusing LEFT JOIN with INNER JOIN
• Assuming unmatched right rows appear in LEFT JOIN

4. You wrote this query but it returns fewer rows than expected:

SELECT * FROM Orders RIGHT JOIN Customers ON Orders.CustomerID = Customers.ID;

What is the likely mistake?

medium

A. Using ON instead of WHERE for join condition

B. Missing WHERE clause to filter rows

C. Not specifying table aliases

D. Using RIGHT JOIN instead of LEFT JOIN when Orders is the main table

Solution

1. Step 1: Identify which table's rows to keep

   If Orders is the main table and you want all its rows, LEFT JOIN should be used, not RIGHT JOIN.

2. Step 2: Understand RIGHT JOIN effect

   RIGHT JOIN keeps all rows from Customers (right table), so if Orders has rows without matching Customers, those rows are excluded.

3. Final Answer:

   Using RIGHT JOIN instead of LEFT JOIN when Orders is the main table -> Option D

4. Quick Check:

   Use LEFT JOIN to keep all left table rows [OK]

Hint: Use LEFT JOIN to keep all rows from first table [OK]

Common Mistakes:

• Confusing LEFT and RIGHT JOIN roles
• Expecting RIGHT JOIN to keep left table rows
• Ignoring join direction impact on row count

5. You have two tables: Students (left) and Enrollments (right). You want a list of all students, including those not enrolled in any course, showing NULL for missing enrollment data.
Which query is best and why?

Option A: SELECT * FROM Students LEFT JOIN Enrollments ON Students.ID = Enrollments.StudentID;
Option B: SELECT * FROM Enrollments RIGHT JOIN Students ON Students.ID = Enrollments.StudentID;

hard

A. Option A, because LEFT JOIN keeps all students and matches enrollments

B. Option B, because RIGHT JOIN keeps all enrollments and matches students

C. Option A, but it excludes students without enrollments

D. Option B, but it excludes students without enrollments

Solution

1. Step 1: Identify which table's rows to keep

   You want all students, even those without enrollments, so keep all rows from Students (left table).

2. Step 2: Choose JOIN type accordingly

   LEFT JOIN keeps all rows from Students and matches enrollments or shows NULL if none. RIGHT JOIN would keep all enrollments, not all students.

3. Final Answer:

   Option A, because LEFT JOIN keeps all students and matches enrollments -> Option A

4. Quick Check:

   LEFT JOIN keeps all left table rows [OK]

Hint: Keep main table first, use LEFT JOIN to keep all its rows [OK]

Common Mistakes:

• Using RIGHT JOIN but expecting all left table rows
• Switching table order without adjusting JOIN type
• Assuming RIGHT JOIN keeps left table rows