FULL OUTER JOIN in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all columns from both tables using a full outer join.

PostgreSQL

SELECT * FROM employees [1] departments ON employees.dept_id = departments.id;

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

AFULL OUTER JOIN

BINNER JOIN

CLEFT JOIN

DRIGHT JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN which only returns matching rows.

Using LEFT JOIN or RIGHT JOIN which return only one side's unmatched rows.

The FULL OUTER JOIN returns all rows when there is a match in one of the tables. It combines the results of both left and right joins.

2fill in blank

medium

Complete the code to join orders and customers tables showing all orders and customers, matching by customer_id.

PostgreSQL

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

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

ARIGHT JOIN

BFULL OUTER JOIN

CINNER JOIN

DLEFT JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN which excludes unmatched rows.

Using LEFT JOIN or RIGHT JOIN which exclude unmatched rows from one side.

Using FULL OUTER JOIN ensures all orders and all customers appear, even if some orders have no customer or some customers have no orders.

3fill in blank

hard

Fix the error in the join clause to correctly perform a full outer join between products and sales.

PostgreSQL

SELECT products.name, sales.amount FROM products [1] sales ON products.id = sales.product_id;

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

AFULL OUTER JOIN

BLEFT OUTER JOIN

CFULL JOIN

DOUTER JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using OUTER JOIN without FULL, LEFT, or RIGHT.

Using LEFT OUTER JOIN which excludes unmatched rows from sales.

The correct syntax for a full outer join in PostgreSQL is FULL OUTER JOIN. Using just FULL JOIN also works but FULL OUTER JOIN is clearer and standard.

4fill in blank

hard

Fill both blanks to create a full outer join that shows all employees and their projects, matching by project_id.

PostgreSQL

SELECT employees.name, projects.title FROM employees [1] projects [2] employees.project_id = projects.id;

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

AFULL OUTER JOIN

BON

CWHERE

DUSING

Attempts:

3 left

💡 Hint

Common Mistakes

Using WHERE instead of ON for join condition.

Using USING when column names differ.

The join type is FULL OUTER JOIN and the condition is specified with ON. The WHERE clause is not used for join conditions here, and USING requires matching column names.

5fill in blank

hard

Fill all three blanks to write a full outer join query that selects employee names, department names, and handles unmatched rows.

PostgreSQL

SELECT employees.name AS employee_name, departments.name AS department_name FROM employees [1] departments [2] employees.dept_id = departments.id [3] employees.name IS NULL OR departments.name IS NULL;

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

AFULL OUTER JOIN

BON

CWHERE

DLEFT JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using LEFT JOIN which excludes unmatched rows from departments.

Using WHERE instead of ON for join condition.

The query uses FULL OUTER JOIN to include all rows. The join condition is specified with ON. The WHERE clause filters rows where either side is unmatched (NULL).

Practice

1. What does a FULL OUTER JOIN do in PostgreSQL?

easy

A. It returns all rows from the right table and matching rows from the left table.

B. It returns only rows that have matching values in both tables.

C. It returns all rows from the left table and matching rows from the right table.

D. It returns all rows from both tables, matching where possible, and NULLs where no match exists.

Solution

1. Step 1: Understand the join types

   INNER JOIN returns only matching rows; LEFT JOIN returns all left table rows plus matches; RIGHT JOIN returns all right table rows plus matches.

2. Step 2: Define FULL OUTER JOIN behavior

   FULL OUTER JOIN returns all rows from both tables, matching where possible, and fills unmatched columns with NULL.

3. Final Answer:

   It returns all rows from both tables, matching where possible, and NULLs where no match exists. -> Option D

4. Quick Check:

   FULL OUTER JOIN = all rows both tables [OK]

Hint: Remember: FULL OUTER JOIN = all rows from both tables [OK]

Common Mistakes:

• Confusing FULL OUTER JOIN with INNER JOIN
• Thinking FULL OUTER JOIN returns only left or right table rows
• Assuming unmatched rows are excluded

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

easy

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

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

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

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

Solution

1. Step 1: Recall PostgreSQL FULL OUTER JOIN syntax

   The correct syntax is: SELECT ... FROM table1 FULL OUTER JOIN table2 ON condition;

2. Step 2: Check each option

   The syntax 'SELECT * FROM table1 FULL OUTER JOIN table2 ON table1.id = table2.id;' matches exactly. Others err: 'JOIN FULL OUTER' wrong order; 'FULL JOIN ... WHERE' misuses WHERE for join; 'OUTER FULL JOIN' wrong sequence.

3. Final Answer:

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

4. Quick Check:

   FULL OUTER JOIN syntax = FULL OUTER JOIN ON [OK]

Hint: FULL OUTER JOIN always uses 'FULL OUTER JOIN ... ON' syntax [OK]

Common Mistakes:

• Swapping JOIN and FULL OUTER keywords
• Omitting ON clause
• Using WHERE instead of ON for join condition

3. Given these tables:
Table A:
ID | Name
1 | Alice
2 | Bob
4 | Diana

Table B:
ID | City
2 | Paris
3 | London
4 | Berlin

What is the result of:
SELECT COALESCE(A.ID, B.ID) AS ID, A.Name, B.City FROM A FULL OUTER JOIN B ON A.ID = B.ID ORDER BY ID;

medium

A. [{2, 'Bob', 'Paris'}, {3, NULL, 'London'}, {4, 'Diana', 'Berlin'}]

B. [{1, 'Alice', NULL}, {2, 'Bob', NULL}, {3, NULL, 'London'}, {4, 'Diana', 'Berlin'}]

C. [{1, 'Alice', NULL}, {2, 'Bob', 'Paris'}, {3, NULL, 'London'}, {4, 'Diana', 'Berlin'}]

D. [{1, 'Alice', NULL}, {2, 'Bob', 'Paris'}, {4, 'Diana', 'Berlin'}]

Solution

1. Step 1: Identify matching and unmatched IDs

   Matching IDs: 2 and 4. Unmatched in A: 1. Unmatched in B: 3.

2. Step 2: Construct FULL OUTER JOIN result

   Rows:
   - ID 1: Name 'Alice', City NULL (no match in B)
   - ID 2: Name 'Bob', City 'Paris' (match)
   - ID 3: Name NULL, City 'London' (no match in A)
   - ID 4: Name 'Diana', City 'Berlin' (match)

3. Final Answer:

   [{1, 'Alice', NULL}, {2, 'Bob', 'Paris'}, {3, NULL, 'London'}, {4, 'Diana', 'Berlin'}] -> Option C

4. Quick Check:

   FULL OUTER JOIN includes all rows with NULLs for unmatched [OK]

Hint: Match IDs, include all rows, NULL for missing side [OK]

Common Mistakes:

• Excluding unmatched rows from either table
• Mixing up NULL placement
• Ordering rows incorrectly

4. Consider this query:
SELECT * FROM A FULL OUTER JOIN B ON A.id = B.id WHERE A.name = 'Alice';
Why might this query return fewer rows than expected?

medium

A. Because the WHERE clause filters out rows where A.name is NULL, excluding unmatched rows from B.

B. Because FULL OUTER JOIN only returns matching rows.

C. Because the ON condition is incorrect syntax.

D. Because the query needs GROUP BY to include all rows.

Solution

1. Step 1: Understand FULL OUTER JOIN with WHERE clause

   FULL OUTER JOIN returns all rows, but WHERE filters after join. Filtering on A.name excludes rows where A.name is NULL (unmatched in A).

2. Step 2: Effect of filtering on unmatched rows

   Rows unmatched in A have NULL for A.name, so WHERE A.name = 'Alice' removes them, reducing rows.

3. Final Answer:

   Because the WHERE clause filters out rows where A.name is NULL, excluding unmatched rows from B. -> Option A

4. Quick Check:

   WHERE filters after join, can exclude NULLs [OK]

Hint: Use WHERE carefully; it filters after join, may exclude NULLs [OK]

Common Mistakes:

• Thinking FULL OUTER JOIN ignores WHERE filters
• Assuming ON clause filters rows after join
• Believing GROUP BY affects join row count

5. You have two tables:
Employees(emp_id, name) and Departments(dept_id, emp_id, dept_name).
You want a list of all employees and their departments if any, plus all departments and their employees if any.
Which query correctly uses FULL OUTER JOIN to achieve this?

hard

A. SELECT e.emp_id, e.name, d.dept_name FROM Employees e RIGHT JOIN Departments d ON e.emp_id = d.emp_id;

B. SELECT e.emp_id, e.name, d.dept_name FROM Employees e FULL OUTER JOIN Departments d ON e.emp_id = d.emp_id;

C. SELECT e.emp_id, e.name, d.dept_name FROM Employees e INNER JOIN Departments d ON e.emp_id = d.emp_id;

D. SELECT e.emp_id, e.name, d.dept_name FROM Employees e LEFT JOIN Departments d ON e.emp_id = d.emp_id;

Solution

1. Step 1: Understand the requirement

   We want all employees and their departments if any, plus all departments and their employees if any. This means all rows from both tables.

2. Step 2: Choose the correct join type

   FULL OUTER JOIN returns all rows from both tables, matching where possible. LEFT or RIGHT JOIN only return all rows from one table.

3. Final Answer:

   SELECT e.emp_id, e.name, d.dept_name FROM Employees e FULL OUTER JOIN Departments d ON e.emp_id = d.emp_id; -> Option B

4. Quick Check:

   FULL OUTER JOIN returns all rows both tables [OK]

Hint: Use FULL OUTER JOIN to include all rows from both tables [OK]

Common Mistakes:

• Using LEFT or RIGHT JOIN excludes unmatched rows from one table
• Using INNER JOIN excludes unmatched rows
• Forgetting ON condition