CROSS JOIN cartesian product in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of CROSS JOIN with two small tables

Given two tables Colors and Shapes, what is the output of the following query?

SELECT Colors.color, Shapes.shape FROM Colors CROSS JOIN Shapes;

Tables:
Colors: {color: 'Red'}, {color: 'Blue'}
Shapes: {shape: 'Circle'}, {shape: 'Square'}

SQL

SELECT Colors.color, Shapes.shape FROM Colors CROSS JOIN Shapes;

A [{'color': 'Red', 'shape': 'Circle'}, {'color': 'Blue', 'shape': 'Square'}]

B [{'color': 'Red', 'shape': 'Circle'}, {'color': 'Red', 'shape': 'Square'}, {'color': 'Blue', 'shape': 'Circle'}, {'color': 'Blue', 'shape': 'Square'}]

C [{'color': 'Red', 'shape': 'Circle'}, {'color': 'Blue', 'shape': 'Circle'}, {'color': 'Red', 'shape': 'Square'}, {'color': 'Blue', 'shape': 'Square'}]

D [{'color': 'Red', 'shape': 'Square'}, {'color': 'Blue', 'shape': 'Circle'}]

💡 Hint

Remember, CROSS JOIN pairs every row from the first table with every row from the second table.

CROSS JOIN produces all combinations of rows from both tables. With 2 colors and 2 shapes, total rows = 2 * 2 = 4.

🧠 Conceptual

intermediate

Understanding the number of rows in a CROSS JOIN

If table A has 5 rows and table B has 3 rows, how many rows will the query SELECT * FROM A CROSS JOIN B; return?

A 5

B 2

C 8

D 15

💡 Hint

Think about how many pairs you can make combining rows from both tables.

CROSS JOIN returns every combination of rows from both tables, so total rows = 5 * 3 = 15.

📝 Syntax

advanced

Identify the correct CROSS JOIN syntax

Which of the following SQL queries correctly performs a CROSS JOIN between tables Employees and Departments?

A SELECT * FROM Employees CROSS JOIN Departments;

B SELECT * FROM Employees JOIN Departments ON Employees.id = Departments.id;

C SELECT * FROM Employees INNER JOIN Departments;

D SELECT * FROM Employees, Departments WHERE Employees.id = Departments.id;

💡 Hint

CROSS JOIN does not require an ON clause.

Option A uses the correct CROSS JOIN syntax. Options B and D use INNER JOIN or implicit join with conditions, which are not CROSS JOIN. Option A is incomplete syntax.

🔧 Debug

advanced

Why does this CROSS JOIN query produce an error?

Consider the query:

SELECT a.name, b.name FROM Authors a CROSS JOIN Books b WHERE a.id = b.author_id;

It produces an error: "ambiguous column name 'name'". Why?

A The query is missing a GROUP BY clause.

B The alias 'a' and 'b' are not allowed in CROSS JOIN queries.

C Both tables have a column named 'name' and the query does not specify which one to use in SELECT.

D CROSS JOIN cannot be used with WHERE clauses.

💡 Hint

Check the column names and how they are referenced in SELECT.

Both Authors and Books tables have a 'name' column. The query selects 'a.name' and 'b.name' which is correct, but if the query was written as SELECT name, name it would be ambiguous. The error suggests the query might have SELECT name, name without aliases.

❓ optimization

expert

Optimizing a CROSS JOIN query with large tables

You have two large tables, Products with 100,000 rows and Colors with 50 rows. You want to list all product-color combinations but the query is slow:

SELECT p.id, c.color FROM Products p CROSS JOIN Colors c;

Which approach can improve performance?

A Filter Products or Colors before the CROSS JOIN to reduce rows.

B Use INNER JOIN instead of CROSS JOIN without any ON condition.

C Add an index on Products.id and Colors.color columns.

D Replace CROSS JOIN with UNION ALL of Products and Colors.

💡 Hint

Think about reducing the number of rows before joining.

Filtering rows before CROSS JOIN reduces the number of combinations and speeds up the query. Indexes don't help much for CROSS JOIN because it produces all combinations. INNER JOIN without ON is invalid. UNION ALL combines rows vertically, not horizontally.

Practice

1. What does a CROSS JOIN do in SQL?

easy

A. It returns all possible combinations of rows from two tables.

B. It returns only matching rows based on a condition.

C. It deletes rows from both tables.

D. It updates rows in one table based on another.

Solution

1. Step 1: Understand the definition of CROSS JOIN

   A CROSS JOIN combines every row from the first table with every row from the second table, creating all possible pairs.

2. Step 2: Compare with other join types

   Unlike INNER JOIN or LEFT JOIN, CROSS JOIN does not require a matching condition and returns the Cartesian product.

3. Final Answer:

   It returns all possible combinations of rows from two tables. -> Option A

4. Quick Check:

   CROSS JOIN = all combinations [OK]

Hint: CROSS JOIN = multiply rows from both tables [OK]

Common Mistakes:

• Confusing CROSS JOIN with INNER JOIN
• Thinking CROSS JOIN filters rows
• Assuming CROSS JOIN deletes or updates data

2. Which of the following is the correct syntax to perform a CROSS JOIN between tables Employees and Departments?

easy

A. SELECT * FROM Employees INNER JOIN Departments;

B. SELECT * FROM Employees JOIN Departments ON Employees.id = Departments.id;

C. SELECT * FROM Employees CROSS JOIN Departments;

D. SELECT * FROM Employees WHERE CROSS JOIN Departments;

Solution

1. Step 1: Identify correct CROSS JOIN syntax

   The correct syntax uses the keyword CROSS JOIN between two table names without any ON condition.

2. Step 2: Eliminate incorrect options

   SELECT * FROM Employees JOIN Departments ON Employees.id = Departments.id; uses JOIN with ON condition (not CROSS JOIN). SELECT * FROM Employees INNER JOIN Departments; uses INNER JOIN without ON, which is invalid. SELECT * FROM Employees WHERE CROSS JOIN Departments; uses WHERE incorrectly.

3. Final Answer:

   SELECT * FROM Employees CROSS JOIN Departments; -> Option C

4. Quick Check:

   Correct CROSS JOIN syntax = SELECT * FROM Employees CROSS JOIN Departments; [OK]

Hint: CROSS JOIN uses keyword CROSS JOIN without ON clause [OK]

Common Mistakes:

• Using JOIN without ON for CROSS JOIN
• Trying to use WHERE for joining tables
• Confusing INNER JOIN with CROSS JOIN syntax

3. Given two tables:
Colors with rows: Red, Blue
Shapes with rows: Circle, Square, Triangle
What will be the result of:
SELECT Colors.color, Shapes.shape FROM Colors CROSS JOIN Shapes;

medium

A. [('Red', 'Circle'), ('Red', 'Square'), ('Red', 'Triangle'), ('Blue', 'Circle'), ('Blue', 'Square'), ('Blue', 'Triangle')]

B. [('Red', 'Circle'), ('Blue', 'Square'), ('Red', 'Triangle')]

C. [('Red', 'Circle'), ('Blue', 'Circle'), ('Red', 'Square'), ('Blue', 'Square')]

D. Empty result set

Solution

1. Step 1: Count rows in each table

   Colors has 2 rows, Shapes has 3 rows.

2. Step 2: Calculate CROSS JOIN result

   CROSS JOIN returns all pairs: 2 * 3 = 6 rows combining each color with each shape.

3. Final Answer:

   [('Red', 'Circle'), ('Red', 'Square'), ('Red', 'Triangle'), ('Blue', 'Circle'), ('Blue', 'Square'), ('Blue', 'Triangle')] -> Option A

4. Quick Check:

   2 colors * 3 shapes = 6 pairs [OK]

Hint: Multiply row counts for CROSS JOIN result size [OK]

Common Mistakes:

• Listing only matching pairs instead of all combinations
• Confusing CROSS JOIN with INNER JOIN
• Ignoring total number of rows in result

4. Consider this SQL query:
SELECT * FROM A CROSS JOIN B WHERE A.id = B.id;
What is the main issue with this query?

medium

A. It will cause a syntax error because CROSS JOIN cannot have WHERE clause.

B. It updates tables A and B unintentionally.

C. It returns no rows because CROSS JOIN filters all rows.

D. It produces the same result as INNER JOIN but less efficiently.

Solution

1. Step 1: Understand CROSS JOIN with WHERE filter

   CROSS JOIN creates all pairs, then WHERE filters pairs where A.id = B.id.

2. Step 2: Compare with INNER JOIN

   This is equivalent to INNER JOIN on A.id = B.id but less efficient because CROSS JOIN generates many unnecessary pairs first.

3. Final Answer:

   It produces the same result as INNER JOIN but less efficiently. -> Option D

4. Quick Check:

   CROSS JOIN + WHERE = INNER JOIN behavior [OK]

Hint: CROSS JOIN + WHERE = INNER JOIN but slower [OK]

Common Mistakes:

• Thinking WHERE is invalid with CROSS JOIN
• Assuming CROSS JOIN filters rows automatically
• Confusing SELECT with UPDATE statements

5. You have two tables:
Products with 4 rows and Colors with 5 rows.
You want to generate a list of all product-color combinations but exclude combinations where the product is discontinued.
Which query correctly uses CROSS JOIN and filtering?

hard

A. SELECT p.name, c.color FROM Products p INNER JOIN Colors c ON p.discontinued = FALSE;

B. SELECT p.name, c.color FROM Products p CROSS JOIN Colors c WHERE p.discontinued = FALSE;

C. SELECT p.name, c.color FROM Products p CROSS JOIN Colors c ON p.discontinued = FALSE;

D. SELECT p.name, c.color FROM Products p, Colors c WHERE p.discontinued = TRUE;

Solution

1. Step 1: Use CROSS JOIN to get all combinations

   Use CROSS JOIN between Products and Colors to get every product-color pair.

2. Step 2: Filter out discontinued products

   Apply WHERE clause to keep only products where discontinued = FALSE.

3. Step 3: Check other options for errors

   SELECT p.name, c.color FROM Products p INNER JOIN Colors c ON p.discontinued = FALSE; uses INNER JOIN incorrectly with ON condition unrelated to join keys. SELECT p.name, c.color FROM Products p CROSS JOIN Colors c ON p.discontinued = FALSE; uses ON with CROSS JOIN, which is invalid syntax. SELECT p.name, c.color FROM Products p, Colors c WHERE p.discontinued = TRUE; filters discontinued = TRUE, opposite of requirement.

4. Final Answer:

   SELECT p.name, c.color FROM Products p CROSS JOIN Colors c WHERE p.discontinued = FALSE; -> Option B

5. Quick Check:

   CROSS JOIN + WHERE filters correctly [OK]

Hint: Filter after CROSS JOIN with WHERE clause [OK]

Common Mistakes:

• Using ON clause with CROSS JOIN
• Filtering with wrong discontinued value
• Confusing INNER JOIN with CROSS JOIN usage