FULL OUTER JOIN availability across databases in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Which databases support FULL OUTER JOIN natively?

Consider these popular SQL databases: MySQL, PostgreSQL, SQL Server, and SQLite. Which of them supports FULL OUTER JOIN as a native SQL feature?

A None support FULL OUTER JOIN natively

B PostgreSQL and SQL Server only

C All four support FULL OUTER JOIN natively

D MySQL and SQLite only

💡 Hint

Think about which databases have FULL OUTER JOIN as a direct keyword in their SQL syntax.

PostgreSQL and SQL Server support FULL OUTER JOIN directly. MySQL and SQLite do not have native FULL OUTER JOIN syntax.

❓ query_result

intermediate

What is the output of this FULL OUTER JOIN query in PostgreSQL?

Given two tables:

Table A:
id | name
1 | Alice
2 | Bob

Table B:
id | city
2 | Paris
3 | Tokyo

What is the result of:

SELECT A.id, A.name, B.city FROM A FULL OUTER JOIN B ON A.id = B.id ORDER BY A.id, B.id;

SQL

CREATE TABLE A (id INT, name TEXT);
INSERT INTO A VALUES (1, 'Alice'), (2, 'Bob');
CREATE TABLE B (id INT, city TEXT);
INSERT INTO B VALUES (2, 'Paris'), (3, 'Tokyo');

SELECT A.id, A.name, B.city FROM A FULL OUTER JOIN B ON A.id = B.id ORDER BY A.id, B.id;

A

1 | Alice | NULL
2 | Bob   | Paris
3 | NULL  | Tokyo

B

1 | Alice | NULL
2 | Bob   | Paris

C

1 | Alice | NULL
2 | Bob   | Paris
NULL | NULL | Tokyo

D

2 | Bob   | Paris
3 | NULL  | Tokyo

💡 Hint

FULL OUTER JOIN returns all rows from both tables, matching where possible.

The FULL OUTER JOIN returns all rows from both tables. For id=1, only A has data, so city is NULL. For id=2, both tables have data. For id=3, only B has data, so name is NULL.

📝 Syntax

advanced

Which query correctly simulates FULL OUTER JOIN in MySQL?

MySQL does not support FULL OUTER JOIN natively. Which of these queries correctly simulates a FULL OUTER JOIN between tables A and B on column id?

A SELECT A.id, A.name, B.city FROM A LEFT JOIN B ON A.id = B.id UNION ALL SELECT B.id, A.name, B.city FROM B LEFT JOIN A ON B.id = A.id WHERE A.id IS NULL;

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

C SELECT A.id, A.name, B.city FROM A INNER JOIN B ON A.id = B.id;

D SELECT A.id, A.name, B.city FROM A LEFT JOIN B ON A.id = B.id UNION SELECT B.id, A.name, B.city FROM B LEFT JOIN A ON B.id = A.id;

💡 Hint

FULL OUTER JOIN can be simulated by combining LEFT JOIN and RIGHT JOIN results with UNION.

Option D uses LEFT JOIN and UNION to combine all rows from both tables, simulating FULL OUTER JOIN. Option D uses UNION ALL but misses filtering correctly. Option D is INNER JOIN only. Option D is invalid syntax in MySQL.

❓ optimization

advanced

Which optimization improves FULL OUTER JOIN performance in SQL Server?

You have a large FULL OUTER JOIN query joining tables A and B on id. Which optimization can improve performance?

A Create indexes on the join columns in both tables

B Use CROSS JOIN instead of FULL OUTER JOIN

C Remove indexes on join columns to speed up scanning

D Use subqueries without indexes

💡 Hint

Indexes help the database find matching rows faster.

Indexes on join columns speed up matching rows in joins. Removing indexes or using CROSS JOIN will degrade performance or change results.

🔧 Debug

expert

Why does this FULL OUTER JOIN query fail in SQLite?

Given this query:

SELECT A.id, B.id FROM A FULL OUTER JOIN B ON A.id = B.id;

Why does it fail in SQLite?

A SQLite does not support FULL OUTER JOIN syntax

B The ON clause is missing a condition

C Table aliases are required in SQLite

D SQLite requires USING instead of ON for joins

💡 Hint

Check SQLite's join capabilities.

SQLite does not support FULL OUTER JOIN syntax at all, so the query fails with a syntax error.

Practice

1. Which of the following SQL databases natively supports the FULL OUTER JOIN clause?

easy

A. PostgreSQL

B. MySQL (before version 8.0)

C. SQLite

D. MariaDB (before version 10.2)

Solution

1. Step 1: Understand FULL OUTER JOIN support in popular databases

   PostgreSQL supports FULL OUTER JOIN natively, allowing retrieval of all matching and non-matching rows from both tables.

2. Step 2: Check other databases' support

   MySQL before 8.0 and SQLite do not support FULL OUTER JOIN natively; MariaDB added support only after version 10.2.

3. Final Answer:

   PostgreSQL -> Option A

4. Quick Check:

   PostgreSQL supports FULL OUTER JOIN [OK]

Hint: Remember PostgreSQL supports FULL OUTER JOIN natively [OK]

Common Mistakes:

• Assuming MySQL supports FULL OUTER JOIN before version 8.0
• Thinking SQLite supports FULL OUTER JOIN
• Confusing MariaDB versions for FULL OUTER JOIN support

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

easy

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

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

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

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

Solution

1. Step 1: Identify correct FULL OUTER JOIN syntax

   The correct syntax uses FULL OUTER JOIN with an ON clause to specify join condition.

2. Step 2: Analyze incorrect options

   SELECT * FROM table1 INNER FULL JOIN table2 ON table1.id = table2.id; uses invalid keyword INNER FULL JOIN, SELECT * FROM table1 FULL JOIN table2 WHERE table1.id = table2.id; misplaces join condition in WHERE, SELECT * FROM table1 OUTER JOIN table2 ON table1.id = table2.id; uses incomplete OUTER JOIN without FULL.

3. Final Answer:

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

4. Quick Check:

   FULL OUTER JOIN syntax = SELECT * FROM table1 FULL OUTER JOIN table2 ON table1.id = table2.id; [OK]

Hint: FULL OUTER JOIN requires ON clause, no INNER keyword [OK]

Common Mistakes:

• Using INNER FULL JOIN instead of FULL OUTER JOIN
• Placing join condition in WHERE instead of ON
• Omitting FULL keyword before OUTER JOIN

3. Given two tables:
Table A: id, name
Table B: id, city
With data:
Table A: (1, 'Alice'), (2, 'Bob')
Table B: (2, 'Paris'), (3, 'London')
What is the result of this query?

SELECT A.id, A.name, B.city FROM A FULL OUTER JOIN B ON A.id = B.id ORDER BY A.id, B.id;

medium

A. (1, 'Alice', NULL), (2, 'Bob', NULL)

B. (1, 'Alice', 'Paris'), (2, 'Bob', 'Paris'), (3, NULL, 'London')

C. (2, 'Bob', 'Paris')

D. (1, 'Alice', NULL), (2, 'Bob', 'Paris'), (3, NULL, 'London')

Solution

1. Step 1: Understand FULL OUTER JOIN behavior

   It returns all rows from both tables, matching rows where possible, NULL where no match.

2. Step 2: Match rows by id and fill NULLs

   id=1 only in A, so city NULL; id=2 in both, so matched; id=3 only in B, so name NULL.

3. Final Answer:

   (1, 'Alice', NULL), (2, 'Bob', 'Paris'), (3, NULL, 'London') -> Option D

4. Quick Check:

   FULL OUTER JOIN returns all rows with NULLs for missing matches [OK]

Hint: FULL OUTER JOIN keeps all rows, fills NULLs for no match [OK]

Common Mistakes:

• Expecting only matched rows like INNER JOIN
• Confusing NULL placement for unmatched rows
• Ignoring rows unique to one table

4. You run this query on MySQL 5.7:

SELECT * FROM employees FULL OUTER JOIN departments ON employees.dept_id = departments.id;

It returns a syntax error. What is the best fix?

medium

A. Upgrade MySQL to version 8.0 or higher

B. Replace FULL OUTER JOIN with LEFT JOIN and UNION with RIGHT JOIN

C. Use CROSS JOIN instead of FULL OUTER JOIN

D. Add parentheses around the JOIN clause

Solution

1. Step 1: Identify MySQL 5.7 limitation

   MySQL 5.7 does not support FULL OUTER JOIN syntax, causing syntax error.

2. Step 2: Use workaround with LEFT JOIN and RIGHT JOIN UNION

   Combine LEFT JOIN and RIGHT JOIN with UNION to simulate FULL OUTER JOIN in MySQL 5.7.

3. Final Answer:

   Replace FULL OUTER JOIN with LEFT JOIN and UNION with RIGHT JOIN -> Option B

4. Quick Check:

   MySQL 5.7 needs LEFT JOIN + RIGHT JOIN UNION for FULL OUTER JOIN [OK]

Hint: MySQL < 8.0 lacks FULL OUTER JOIN; use LEFT + RIGHT JOIN UNION [OK]

Common Mistakes:

• Trying to use FULL OUTER JOIN directly in MySQL 5.7
• Using CROSS JOIN which returns Cartesian product
• Thinking parentheses fix syntax error

5. You want to write a query that returns all customers and all orders, matching by customer_id, but your database is SQLite which does not support FULL OUTER JOIN. Which approach correctly simulates FULL OUTER JOIN?

hard

A. Use CROSS JOIN and filter rows where customer_id matches

B. Use INNER JOIN customers to orders and add WHERE clause for unmatched rows

C. Use LEFT JOIN customers to orders UNION ALL RIGHT JOIN orders to customers with filtering duplicates

D. Use LEFT JOIN only from customers to orders

Solution

1. Step 1: Recognize SQLite limitation

   SQLite does not support FULL OUTER JOIN, so we must simulate it.

2. Step 2: Simulate FULL OUTER JOIN with UNION of LEFT and RIGHT JOIN

   Use LEFT JOIN customers to orders plus UNION ALL with RIGHT JOIN orders to customers (emulated by LEFT JOIN orders to customers) filtering duplicates to get all rows.

3. Final Answer:

   Use LEFT JOIN customers to orders UNION ALL RIGHT JOIN orders to customers with filtering duplicates -> Option C

4. Quick Check:

   Simulate FULL OUTER JOIN with LEFT + RIGHT JOIN UNION [OK]

Hint: Simulate FULL OUTER JOIN with LEFT JOIN UNION RIGHT JOIN [OK]

Common Mistakes:

• Using INNER JOIN which excludes unmatched rows
• Using CROSS JOIN which creates all combinations
• Using only LEFT JOIN missing unmatched rows from right table