FULL OUTER JOIN behavior in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does a FULL OUTER JOIN do in SQL?

It returns all rows from both tables, matching rows where possible. If there is no match, it fills with NULLs for the missing side.

beginner

How does FULL OUTER JOIN differ from INNER JOIN?

INNER JOIN returns only rows with matching keys in both tables. FULL OUTER JOIN returns all rows from both tables, matching where possible and filling NULLs where no match exists.

beginner

In a FULL OUTER JOIN, what happens if a row in the left table has no matching row in the right table?

The row from the left table appears in the result with NULL values for the right table's columns.

beginner

Write a simple SQL query using FULL OUTER JOIN between tables A and B on column id.

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

intermediate

Why might you use FULL OUTER JOIN instead of LEFT or RIGHT JOIN?

To get a complete view of all data from both tables, including unmatched rows from both sides.

What does FULL OUTER JOIN return?

A All rows from the left table only

B Only matching rows from both tables

C All rows from the right table only

D All rows from both tables, matching where possible, NULLs otherwise

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

If a row in the right table has no match in the left table, what does FULL OUTER JOIN do?

A Excludes the row

B Includes the row with NULLs for left table columns

C Includes the row with NULLs for right table columns

D Returns an error

FULL OUTER JOIN includes unmatched rows from both tables, filling NULLs for the missing side's columns.

Which JOIN type returns only rows with matching keys in both tables?

A FULL OUTER JOIN

B LEFT JOIN

C INNER JOIN

D RIGHT JOIN

INNER JOIN returns only rows where keys match in both tables.

What keyword is used in SQL for a FULL OUTER JOIN?

A FULL OUTER JOIN

B ALL JOIN

C OUTER JOIN

D FULL JOIN

The correct syntax is FULL OUTER JOIN to include all rows from both tables.

Which scenario best fits using FULL OUTER JOIN?

A You want all records from both tables, including unmatched

B You want all records from left table only

C You want only matching records

D You want only unmatched records

FULL OUTER JOIN returns all records from both tables, including unmatched rows.

Explain in your own words how FULL OUTER JOIN works and when you would use it.

Think about combining two lists fully, including unmatched items.

Write a simple SQL query using FULL OUTER JOIN and describe what the output will look like.

Use SELECT * FROM table1 FULL OUTER JOIN table2 ON condition.

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