LEFT JOIN and RIGHT JOIN in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all customers and their orders, including customers without orders.

PostgreSQL

SELECT customers.name, orders.order_id FROM customers [1] orders ON customers.id = orders.customer_id;

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

ALEFT JOIN

BRIGHT JOIN

CINNER JOIN

DFULL JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN excludes customers without orders.

RIGHT JOIN would keep all orders, not customers.

LEFT JOIN returns all rows from the left table (customers) and matching rows from the right table (orders). Customers without orders still appear.

2fill in blank

medium

Complete the code to select all orders and their customers, including orders without customers.

PostgreSQL

SELECT orders.order_id, customers.name FROM orders [1] customers ON orders.customer_id = customers.id;

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

AFULL JOIN

BINNER JOIN

CLEFT JOIN

DRIGHT JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN excludes orders without customers.

Using LEFT JOIN would keep all orders, not customers.

RIGHT JOIN returns all rows from the right table (customers) and matching rows from the left table (orders). Orders without customers still appear.

3fill in blank

hard

Fix the error in the join to include all employees and their departments, even if some employees have no department.

PostgreSQL

SELECT employees.name, departments.dept_name FROM employees [1] departments ON employees.dept_id = departments.id;

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

AINNER JOIN

BLEFT JOIN

CRIGHT JOIN

DCROSS JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN excludes employees without departments.

RIGHT JOIN keeps all departments, not employees.

LEFT JOIN keeps all employees (left table) even if they have no matching department.

4fill in blank

hard

Fill both blanks to select all products and their suppliers, including products without suppliers.

PostgreSQL

SELECT products.name, suppliers.name FROM products [1] suppliers ON products.supplier_id [2] suppliers.id;

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

ALEFT JOIN

B=

C!=

DRIGHT JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using RIGHT JOIN would keep all suppliers, not products.

Using '!=' in ON condition is incorrect for matching.

LEFT JOIN keeps all products even if they have no supplier. The ON condition uses '=' to match supplier IDs.

5fill in blank

hard

Fill all three blanks to select all orders, their customers, and their shipping info, including orders without customers or shipping info.

PostgreSQL

SELECT orders.id, customers.name, shipping.address FROM orders [1] customers ON orders.customer_id [2] customers.id [3] JOIN shipping ON orders.shipping_id = shipping.id;

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

ALEFT JOIN

B=

CLEFT

DINNER

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN excludes orders without customers or shipping info.

Using '!=' in ON condition is incorrect.

LEFT JOIN keeps all orders even if no matching customer. The ON condition uses '='. The shipping table is joined with LEFT JOIN to keep orders without shipping info.

Practice

1. What does a LEFT JOIN do in PostgreSQL?

easy

A. Returns all rows from the left table and matching rows from the right table, or NULL if no match.

B. Returns all rows from the right table and matching rows from the left table, or NULL if no match.

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 LEFT JOIN behavior

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

2. Step 2: Check what happens with unmatched rows

   If there is no matching row in the right table, the result includes NULLs for right table columns.

3. Final Answer:

   Returns all rows from the left table and matching rows from the right table, or NULL if no match. -> Option A

4. Quick Check:

   LEFT JOIN = all left rows + matched right rows [OK]

Hint: LEFT JOIN keeps all left rows, fills right with NULL if no match [OK]

Common Mistakes:

• Confusing LEFT JOIN with RIGHT JOIN
• Thinking LEFT JOIN returns only matched rows
• Assuming unmatched rows are dropped

2. Which of the following is the correct syntax for a RIGHT JOIN in PostgreSQL?

easy

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

B. SELECT * FROM table1 JOIN RIGHT table2 ON table1.id = table2.id;

C. SELECT * FROM table1 RIGHT OUTER JOIN table2 WHERE table1.id = table2.id;

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

Solution

1. Step 1: Recall correct RIGHT JOIN syntax

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

2. Step 2: Check each option

   SELECT * FROM table1 RIGHT JOIN table2 ON table1.id = table2.id; matches the correct syntax exactly. Options B, C, and D have syntax errors or misplaced keywords.

3. Final Answer:

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

4. Quick Check:

   RIGHT JOIN syntax = FROM left_table RIGHT JOIN right_table ON condition [OK]

Hint: RIGHT JOIN syntax: FROM left_table RIGHT JOIN right_table ON condition [OK]

Common Mistakes:

• Placing JOIN keyword incorrectly
• Using WHERE instead of ON for join condition
• Misordering table names

3. Given these tables:

employees
id | name
1 | Alice
2 | Bob
3 | Carol

departments
id | dept_name
2 | Sales
3 | HR
4 | IT

What is the result of this query?

SELECT employees.name, departments.dept_name FROM employees LEFT JOIN departments ON employees.id = departments.id ORDER BY employees.id;

medium

A. [{"name": "Alice", "dept_name": "Sales"}, {"name": "Bob", "dept_name": "HR"}, {"name": "Carol", "dept_name": "IT"}]

B. [{"name": "Bob", "dept_name": "Sales"}, {"name": "Carol", "dept_name": "HR"}, {"name": null, "dept_name": "IT"}]

C. [{"name": "Alice", "dept_name": null}, {"name": "Bob", "dept_name": "Sales"}, {"name": "Carol", "dept_name": "HR"}]

D. [{"name": "Alice", "dept_name": null}, {"name": "Bob", "dept_name": null}, {"name": "Carol", "dept_name": null}]

Solution

1. Step 1: Understand LEFT JOIN with given tables

   LEFT JOIN keeps all employees. For each employee, it tries to find a department with matching id.

2. Step 2: Match rows and fill NULLs

   Alice (id=1) has no matching department, so dept_name is NULL. Bob (id=2) matches Sales. Carol (id=3) matches HR.

3. Final Answer:

   [{"name": "Alice", "dept_name": null}, {"name": "Bob", "dept_name": "Sales"}, {"name": "Carol", "dept_name": "HR"}] -> Option C

4. Quick Check:

   LEFT JOIN keeps all employees, unmatched dept_name is NULL [OK]

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

Common Mistakes:

• Assuming unmatched rows are excluded
• Mixing up LEFT and RIGHT JOIN results
• Ignoring NULL values for unmatched rows

4. You wrote this query but it gives an error:

SELECT * FROM employees RIGHT JOIN departments employees.id = departments.id;

What is the error and how to fix it?

medium

A. RIGHT JOIN is not supported; use LEFT JOIN instead

B. Missing ON keyword before join condition; fix by adding ON before employees.id = departments.id

C. Table names are reversed; switch employees and departments

D. Use WHERE instead of ON for join condition

Solution

1. Step 1: Identify syntax error in JOIN clause

   The query is missing the ON keyword before the join condition, which is required syntax.

2. Step 2: Correct the syntax

   Add ON before employees.id = departments.id to fix the error.

3. Final Answer:

   Missing ON keyword before join condition; fix by adding ON before employees.id = departments.id -> Option B

4. Quick Check:

   JOIN condition must use ON keyword [OK]

Hint: JOIN conditions always need ON keyword before condition [OK]

Common Mistakes:

• Omitting ON keyword
• Using WHERE instead of ON for join condition
• Confusing table order in JOIN

5. You want to list all departments and their employees, including departments with no employees. Which query achieves this?

hard

A. SELECT departments.dept_name, employees.name FROM employees RIGHT JOIN departments WHERE employees.id = departments.id;

B. SELECT departments.dept_name, employees.name FROM employees LEFT JOIN departments ON employees.id = departments.id;

C. SELECT departments.dept_name, employees.name FROM departments RIGHT JOIN employees ON departments.id = employees.id;

D. SELECT departments.dept_name, employees.name FROM departments LEFT JOIN employees ON departments.id = employees.id;

Solution

1. Step 1: Identify which table to keep all rows from

   We want all departments, even those without employees, so departments is the left table in LEFT JOIN.

2. Step 2: Use LEFT JOIN from departments to employees

   LEFT JOIN keeps all departments and adds matching employees or NULL if none.

3. Final Answer:

   SELECT departments.dept_name, employees.name FROM departments LEFT JOIN employees ON departments.id = employees.id; -> Option D

4. Quick Check:

   LEFT JOIN from departments keeps all departments [OK]

Hint: LEFT JOIN from main table keeps all its rows, unmatched get NULLs [OK]

Common Mistakes:

• Using employees as left table instead of departments
• Using RIGHT JOIN incorrectly
• Confusing which table to keep all rows from