FULL OUTER JOIN behavior in SQL - Step-by-Step Execution

Concept Flow - FULL OUTER JOIN behavior

Start with Table A and Table B

↓

Match rows on join condition

↓

Combine matched rows into result

↓

Add unmatched rows from Table A with NULLs for B

↓

Add unmatched rows from Table B with NULLs for A

↓

Final Result Set

FULL OUTER JOIN returns all rows from both tables, matching rows where possible, and fills NULLs where no match exists.

Execution Sample

SQL

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

This query joins tables A and B on id, returning all rows from both tables with NULLs where no match exists.

Execution Table

Step	Table A Row	Table B Row	Join Condition (A.id = B.id)	Action	Output Row
1	A.id=1, A.value='Apple'	B.id=1, B.value='Red'	True	Combine matched rows	1, 'Apple', 1, 'Red'
2	A.id=2, A.value='Banana'	B.id=2, B.value='Yellow'	True	Combine matched rows	2, 'Banana', 2, 'Yellow'
3	A.id=3, A.value='Cherry'	No matching B row	False	Add A row with NULLs for B	3, 'Cherry', NULL, NULL
4	No matching A row	B.id=4, B.value='Green'	False	Add B row with NULLs for A	NULL, NULL, 4, 'Green'
5	No more rows	No more rows	N/A	End of join	Result complete

💡 All rows from both tables processed; unmatched rows included with NULLs.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
Output Rows Count	0	1	2	3	4	4

Key Moments - 2 Insights

Why do some columns have NULL values in the output?

Because FULL OUTER JOIN includes unmatched rows from both tables, it fills missing side columns with NULLs as shown in steps 3 and 4 of the execution_table.

Does FULL OUTER JOIN only return rows where the join condition is true?

No, it returns all rows from both tables. Matched rows are combined (steps 1 and 2), but unmatched rows from either table are also included with NULLs (steps 3 and 4).

Visual Quiz - 3 Questions

Look at the execution_table, what is the output row at step 3?

A NULL, NULL, 3, 'Cherry'

B 3, 'Cherry', NULL, NULL

C 3, 'Cherry', 3, 'Cherry'

D NULL, NULL, NULL, NULL

💡 Hint

Check the 'Output Row' column for step 3 in the execution_table.

At which step does the join condition evaluate to False because there is no matching row in Table B?

A Step 1

B Step 4

C Step 3

D Step 2

💡 Hint

Look at the 'Join Condition' column in the execution_table for where Table B row is missing.

If Table B had an extra row with id=5, how would the output rows count change after step 4?

A It would increase by 1

B It would stay the same

C It would decrease by 1

D It would double

💡 Hint

Refer to variable_tracker for how output rows count increases with unmatched rows from Table B.

Concept Snapshot

FULL OUTER JOIN syntax:
SELECT columns
FROM A
FULL OUTER JOIN B ON A.key = B.key;

Returns all rows from A and B.
Matches rows where keys equal.
Unmatched rows show NULLs for missing side.

Full Transcript

FULL OUTER JOIN returns all rows from both tables involved in the join. It matches rows where the join condition is true and combines their columns. For rows in one table without a match in the other, it includes them with NULL values for the missing columns. This ensures no data is lost from either table. The execution steps show matching rows combined first, then unmatched rows from each table added with NULLs. The output row count increases with each added row. This join is useful when you want a complete view of both tables, including unmatched data.

Practice

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

easy

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

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

C. Returns all rows from both tables, matching where possible and NULLs where no match exists.

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

Solution

1. Step 1: Understand join types

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

2. Step 2: Define FULL OUTER JOIN behavior

   FULL OUTER JOIN returns all rows from both tables, matching where possible, and fills NULLs where no match exists.

3. Final Answer:

   Returns all rows from both tables, matching where possible and NULLs where no match exists. -> Option C

4. Quick Check:

   FULL OUTER JOIN = all rows both tables [OK]

Hint: Full outer join keeps all rows from both tables [OK]

Common Mistakes:

• Confusing FULL OUTER JOIN with INNER JOIN
• Thinking FULL OUTER JOIN returns only matches
• Mixing up LEFT and RIGHT JOIN behavior

2. Which of the following is the correct syntax for a FULL OUTER JOIN between tables Employees and Departments on DeptID?

easy

A. SELECT * FROM Employees FULL OUTER JOIN Departments ON Employees.DeptID = Departments.DeptID;

B. SELECT * FROM Employees INNER JOIN Departments ON Employees.DeptID = Departments.DeptID;

C. SELECT * FROM Employees LEFT JOIN Departments ON Employees.DeptID = Departments.DeptID;

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

Solution

1. Step 1: Identify FULL OUTER JOIN syntax

   The correct syntax uses the keywords FULL OUTER JOIN between the two tables with an ON condition.

2. Step 2: Compare options

   SELECT * FROM Employees FULL OUTER JOIN Departments ON Employees.DeptID = Departments.DeptID; uses FULL OUTER JOIN correctly; others use INNER, LEFT, or RIGHT JOIN which are different join types.

3. Final Answer:

   SELECT * FROM Employees FULL OUTER JOIN Departments ON Employees.DeptID = Departments.DeptID; -> Option A

4. Quick Check:

   FULL OUTER JOIN syntax = SELECT * FROM Employees FULL OUTER JOIN Departments ON Employees.DeptID = Departments.DeptID; [OK]

Hint: FULL OUTER JOIN syntax includes FULL OUTER JOIN keywords [OK]

Common Mistakes:

• Using INNER JOIN instead of FULL OUTER JOIN
• Omitting FULL keyword and writing only OUTER JOIN
• Confusing LEFT or RIGHT JOIN with FULL OUTER JOIN

3. Given these tables:

Table A:
ID | Name
1 | Alice
2 | Bob
4 | Dana

Table B:
ID | City
2 | Boston
3 | Chicago
4 | Denver

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;

medium

A. [ (1, 'Alice', NULL), (2, 'Bob', NULL), (3, NULL, 'Chicago'), (4, 'Dana', NULL) ]

B. [ (2, 'Bob', 'Boston'), (4, 'Dana', 'Denver') ]

C. [ (1, 'Alice', 'Boston'), (2, 'Bob', 'Chicago'), (3, NULL, 'Denver'), (4, 'Dana', NULL) ]

D. [ (1, 'Alice', NULL), (2, 'Bob', 'Boston'), (4, 'Dana', 'Denver'), (NULL, NULL, 'Chicago') ]

Solution

1. Step 1: Match rows by ID using FULL OUTER JOIN

   IDs 2 and 4 appear in both tables, so their rows combine. ID 1 is only in A, ID 3 only in B.

2. Step 2: Fill NULLs for missing matches

   For ID 1 (A only), City is NULL; for ID 3 (B only), A.ID=NULL, A.Name=NULL, City='Chicago'. ORDER BY A.ID ASC places NULL last: rows for 1,2,4 then NULL row.

3. Final Answer:

   [ (1, 'Alice', NULL), (2, 'Bob', 'Boston'), (4, 'Dana', 'Denver'), (NULL, NULL, 'Chicago') ] -> Option D

4. Quick Check:

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

Hint: FULL OUTER JOIN shows all rows, NULLs where no match [OK]

Common Mistakes:

• Ignoring unmatched rows from either table
• Assuming INNER JOIN behavior returns only matches
• Misordering results by A.ID when NULLs exist

4. Consider this query:

SELECT * FROM Customers FULL OUTER JOIN Orders ON Customers.CustomerID = Orders.CustomerID WHERE Orders.OrderID IS NULL;

What is the likely purpose of this query?

medium

A. To find orders without customers.

B. To find customers who have no orders.

C. To find all customers and orders regardless of match.

D. To find customers with at least one order.

Solution

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

   The join returns all customers and orders; filtering WHERE Orders.OrderID IS NULL keeps rows where no matching order exists.

2. Step 2: Interpret the filter effect

   Rows with NULL in Orders.OrderID mean customers without orders are selected.

3. Final Answer:

   To find customers who have no orders. -> Option B

4. Quick Check:

   Filter NULL in Orders = customers without orders [OK]

Hint: Filter NULL in joined table to find unmatched rows [OK]

Common Mistakes:

• Thinking it finds orders without customers
• Assuming it returns all rows without filtering
• Confusing NULL filter with matching rows

5. You have two tables:

Products:
ProductID | Name
1 | Pen
2 | Pencil
3 | Eraser

Sales:
ProductID | Quantity
2 | 100
3 | 50
4 | 10

You want a query to list all products and sales quantities, including products with no sales and sales for unknown products.

Which query correctly achieves this?

hard

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

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

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

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

Solution

1. Step 1: Identify requirement for all products and all sales

   We want all products even if no sales, and all sales even if product unknown.

2. Step 2: Choose join type

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

3. Step 3: Check options

   SELECT Products.ProductID, Products.Name, Sales.Quantity FROM Products FULL OUTER JOIN Sales ON Products.ProductID = Sales.ProductID; uses FULL OUTER JOIN correctly; others exclude unmatched rows from one side.

4. Final Answer:

   SELECT Products.ProductID, Products.Name, Sales.Quantity FROM Products FULL OUTER JOIN Sales ON Products.ProductID = Sales.ProductID; -> Option A

5. Quick Check:

   FULL OUTER JOIN = all products and sales [OK]

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

Common Mistakes:

• Using LEFT JOIN excludes sales without products
• Using INNER JOIN excludes unmatched rows
• Confusing RIGHT JOIN direction with LEFT JOIN