FULL OUTER JOIN availability across databases in SQL - Step-by-Step Execution

Concept Flow - FULL OUTER JOIN availability across databases

Start with two tables

↓

Check matching rows

↓

Include matched rows

↓

Include unmatched rows from left

↓

Include unmatched rows from right

↓

Combine all rows

↓

Result: FULL OUTER JOIN

FULL OUTER JOIN combines all rows from two tables, matching where possible and including unmatched rows from both sides.

Execution Sample

SQL

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

This query returns all rows from tables A and B, matching rows where ids are equal, and including unmatched rows from both tables.

Execution Table

Step	Action	Table A Row	Table B Row	Result Row Included	Reason
1	Check A.id=1 with B.id=1	A.id=1	B.id=1	Yes	Match found, include combined row
2	Check A.id=2 with B.id=2	A.id=2	B.id=2	Yes	Match found, include combined row
3	Check A.id=3 with B.id=3	A.id=3	No matching B row	Yes	No match in B, include A row with NULLs for B
4	Check B.id=4 with A.id=4	No matching A row	B.id=4	Yes	No match in A, include B row with NULLs for A
5	No more rows	N/A	N/A	No	All rows processed, stop

💡 All rows from both tables checked and included if matched or unmatched.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
Result Rows Included	0	1	2	3	4	4

Key Moments - 2 Insights

Why do unmatched rows from both tables appear in the result?

Because FULL OUTER JOIN includes all rows from both tables, matched or not, as shown in execution_table rows 3 and 4.

What happens if a database does not support FULL OUTER JOIN?

You cannot directly use FULL OUTER JOIN syntax; you must simulate it using UNION of LEFT JOIN and RIGHT JOIN, unlike the direct inclusion shown in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, how many rows are included in the result after step 3?

A 4

B 2

C 3

D 1

💡 Hint

Check the 'Result Row Included' column up to step 3.

At which step does the join include a row from table B that has no matching row in table A?

A Step 2

B Step 4

C Step 3

D Step 1

💡 Hint

Look for a step where 'Table A Row' is 'No matching A row' and 'Table B Row' has a value.

If a database does not support FULL OUTER JOIN, which approach can simulate it?

A Use UNION of LEFT JOIN and RIGHT JOIN

B Use CROSS JOIN

C Use INNER JOIN only

D Use only LEFT JOIN

💡 Hint

Refer to key_moments explanation about simulating FULL OUTER JOIN.

Concept Snapshot

FULL OUTER JOIN returns all rows from both tables.
Matched rows are combined.
Unmatched rows from either table appear with NULLs.
Not all databases support it directly.
Simulate with LEFT JOIN UNION RIGHT JOIN if needed.

Full Transcript

FULL OUTER JOIN combines two tables by including all rows from both, matching rows where possible and including unmatched rows from either side with NULLs for missing data. The example query selects all rows from tables A and B joined on id. The execution table shows step-by-step how matched and unmatched rows are included. Some databases do not support FULL OUTER JOIN syntax directly; in those cases, you can simulate it by combining LEFT JOIN and RIGHT JOIN results with UNION. This ensures you get the full set of rows from both tables, just like FULL OUTER JOIN would provide.

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