RIGHT JOIN execution behavior in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all columns from table1 and matching rows from table2 using a RIGHT JOIN.

SQL

SELECT * FROM table1 [1] table2 ON table1.id = table2.id;

Drag options to blanks, or click blank then click option'

ARIGHT JOIN

BLEFT JOIN

CINNER JOIN

DFULL JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using LEFT JOIN instead of RIGHT JOIN.

Using INNER JOIN which only returns matching rows.

The RIGHT JOIN returns all rows from the right table (table2), and the matched rows from the left table (table1). If there is no match, NULLs appear for left table columns.

2fill in blank

medium

Complete the code to get all rows from the right table and matching rows from the left table where the right table's status is 'active'.

SQL

SELECT * FROM users [1] orders ON users.user_id = orders.user_id WHERE orders.status = 'active';

Drag options to blanks, or click blank then click option'

ALEFT JOIN

BRIGHT JOIN

CINNER JOIN

DCROSS JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN which excludes unmatched rows.

Using LEFT JOIN which returns all rows from the left table.

The RIGHT JOIN ensures all rows from orders (right table) are included, even if no matching users row exists. The WHERE clause filters for active orders.

3fill in blank

hard

Fix the error in the query to correctly perform a RIGHT JOIN between employees and departments on department_id.

SQL

SELECT employees.name, departments.name FROM employees [1] departments ON employees.department_id = departments.department_id;

Drag options to blanks, or click blank then click option'

ALEFT JOIN

BJOIN

CRIGHT JOIN

DFULL JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using JOIN or INNER JOIN which excludes unmatched departments.

Using LEFT JOIN which returns all employees instead.

The query needs a RIGHT JOIN to include all departments even if no employee belongs to them.

4fill in blank

hard

Fill both blanks to select all customers and their orders, including customers without orders, using RIGHT JOIN and aliasing.

SQL

SELECT o.order_id, c.customer_name FROM orders AS [1] [2] customers AS c ON o.customer_id = c.customer_id;

Drag options to blanks, or click blank then click option'

Ao

BLEFT JOIN

CRIGHT JOIN

Dord

Attempts:

3 left

💡 Hint

Common Mistakes

Using LEFT JOIN instead of RIGHT JOIN.

Not aliasing the orders table correctly.

Alias orders as o and use RIGHT JOIN to include all customers even if they have no orders.

5fill in blank

hard

Fill all three blanks to create a RIGHT JOIN query that selects product names and their categories, including categories without products.

SQL

SELECT p.product_name, c.category_name FROM products AS [1] [2] categories AS [3] ON p.category_id = c.category_id;

Drag options to blanks, or click blank then click option'

Ap

BRIGHT JOIN

Cc

DLEFT JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using LEFT JOIN which excludes categories without products.

Not aliasing categories as 'c'.

Alias products as p, use RIGHT JOIN to include all categories, and alias categories as c.

Practice

1. What does a RIGHT JOIN do in SQL?

easy

A. Returns all rows from the right table and matching rows from the left table.

B. Returns all rows from the left table and matching rows from the right table.

C. Returns only rows that have matching values in both tables.

D. Returns all rows from both tables, matching where possible.

Solution

1. Step 1: Understand RIGHT JOIN behavior

   A RIGHT JOIN returns all rows from the right table regardless of matches in the left table.

2. Step 2: Identify matching rows from the left table

   It includes matching rows from the left table and fills NULL where no match exists.

3. Final Answer:

   Returns all rows from the right table and matching rows from the left table. -> Option A

4. Quick Check:

   RIGHT JOIN = all right table rows + matched left rows [OK]

Hint: Remember: RIGHT JOIN keeps all right table rows [OK]

Common Mistakes:

• Confusing RIGHT JOIN with LEFT JOIN
• Thinking it returns only matching rows
• Assuming it returns all rows from left table

2. Which of the following is the correct syntax for a RIGHT JOIN between tables Employees and Departments on DepartmentID?

easy

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

B. SELECT * FROM Employees JOIN Departments RIGHT ON Employees.DepartmentID = Departments.DepartmentID;

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

D. SELECT * FROM Employees RIGHT JOIN Departments ON Employees.DepartmentID = Departments.DepartmentID;

Solution

1. Step 1: Identify correct JOIN syntax

   The correct syntax is: FROM left_table RIGHT JOIN right_table ON condition.

2. Step 2: Match the syntax with given options

   SELECT * FROM Employees RIGHT JOIN Departments ON Employees.DepartmentID = Departments.DepartmentID; correctly uses RIGHT JOIN with ON clause and proper table order.

3. Final Answer:

   SELECT * FROM Employees RIGHT JOIN Departments ON Employees.DepartmentID = Departments.DepartmentID; -> Option D

4. Quick Check:

   RIGHT JOIN syntax = FROM left RIGHT JOIN right ON condition [OK]

Hint: RIGHT JOIN syntax: FROM left RIGHT JOIN right ON condition [OK]

Common Mistakes:

• Placing RIGHT keyword after JOIN
• Using WHERE instead of ON for join condition
• Incorrect table order in JOIN

3. Given tables:
Employees:
ID | Name | DeptID
1 | Alice | 10
2 | Bob | 20
3 | Carol | NULL

Departments:
DeptID | DeptName
10 | Sales
20 | Marketing
30 | HR

What is the result of:
SELECT Employees.Name, Departments.DeptName FROM Employees RIGHT JOIN Departments ON Employees.DeptID = Departments.DeptID;?

medium

A. [('Alice', 'Sales'), ('Bob', 'Marketing'), (NULL, 'HR')]

B. [('Alice', 'Sales'), ('Bob', 'Marketing')]

C. [('Alice', 'Sales'), ('Bob', 'Marketing'), ('Carol', NULL)]

D. [('Alice', 'Sales'), ('Bob', 'Marketing'), ('Carol', 'HR')]

Solution

1. Step 1: Identify RIGHT JOIN effect on rows

   RIGHT JOIN keeps all Departments rows (right table), matching Employees rows or NULL if no match.

2. Step 2: Match Employees to Departments by DeptID

   DeptID 10 matches Alice, 20 matches Bob, 30 has no employee so NULL for Name.

3. Final Answer:

   [('Alice', 'Sales'), ('Bob', 'Marketing'), (NULL, 'HR')] -> Option A

4. Quick Check:

   RIGHT JOIN keeps all right rows, unmatched left columns NULL [OK]

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

Common Mistakes:

• Ignoring unmatched right table rows
• Assuming unmatched left rows appear
• Mixing up NULL placement

4. Consider this SQL query:
SELECT * FROM Orders RIGHT JOIN Customers ON Orders.CustomerID = Customers.ID;
It returns fewer rows than expected. What is a likely cause?

medium

A. RIGHT JOIN always returns fewer rows than LEFT JOIN.

B. Orders table is empty, so no rows are returned.

C. The JOIN condition uses wrong column names causing no matches.

D. RIGHT JOIN syntax requires WHERE instead of ON clause.

Solution

1. Step 1: Analyze JOIN condition correctness

   If column names in ON clause are wrong, no matches occur, reducing rows.

2. Step 2: Understand RIGHT JOIN behavior with no matches

   RIGHT JOIN still returns all right table rows, but if condition is wrong, matches fail and left columns are NULL.

3. Final Answer:

   The JOIN condition uses wrong column names causing no matches. -> Option C

4. Quick Check:

   Wrong ON columns cause fewer matches [OK]

Hint: Check ON clause column names carefully [OK]

Common Mistakes:

• Assuming RIGHT JOIN returns fewer rows by default
• Confusing ON and WHERE clauses
• Ignoring empty tables impact

5. You have two tables:
Products:
ProductID | Name
1 | Pen
2 | Pencil
3 | Eraser

Sales:
SaleID | ProductID | Quantity
101 | 1 | 10
102 | 2 | 5

You want a report showing all products and their sold quantities, including products with no sales (show quantity as 0). Which query correctly uses RIGHT JOIN and handles missing sales?

hard

A. SELECT Products.Name, Sales.Quantity FROM Products RIGHT JOIN Sales ON Products.ProductID = Sales.ProductID;

B. SELECT Products.Name, COALESCE(Sales.Quantity, 0) AS Quantity FROM Sales RIGHT JOIN Products ON Sales.ProductID = Products.ProductID;

C. SELECT Products.Name, COALESCE(Sales.Quantity, 0) AS Quantity FROM Products LEFT JOIN Sales ON Products.ProductID = Sales.ProductID;

D. SELECT Products.Name, Sales.Quantity FROM Sales LEFT JOIN Products ON Sales.ProductID = Products.ProductID;

Solution

1. Step 1: Identify which table is right and which is left

   Products is the right table to keep all products; Sales is left table.

2. Step 2: Use RIGHT JOIN from Sales to Products and handle NULLs

   RIGHT JOIN keeps all Products rows; COALESCE replaces NULL sales quantity with 0.

3. Final Answer:

   SELECT Products.Name, COALESCE(Sales.Quantity, 0) AS Quantity FROM Sales RIGHT JOIN Products ON Sales.ProductID = Products.ProductID; -> Option B

4. Quick Check:

   RIGHT JOIN keeps all right rows; COALESCE handles NULLs [OK]

Hint: Use COALESCE to replace NULLs after RIGHT JOIN [OK]

Common Mistakes:

• Using LEFT JOIN instead of RIGHT JOIN
• Not handling NULL sales quantities
• Swapping table order in JOIN