CROSS JOIN behavior in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of CROSS JOIN with two small tables

Given two tables Colors and Shapes with the following rows:

Colors:
id | name
1 | Red
2 | Blue

Shapes:
id | name
1 | Circle
2 | Square

What is the output of the query:

SELECT Colors.name AS color, Shapes.name AS shape FROM Colors CROSS JOIN Shapes ORDER BY color, shape;

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

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

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

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

💡 Hint

Remember that 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. Since we order by color then shape, Blue rows come before Red rows alphabetically, so Blue pairs appear first.

🧠 Conceptual

intermediate

Understanding row count after CROSS JOIN

If table A has 5 rows and table B has 3 rows, how many rows will the result have after performing a CROSS JOIN between A and B?

A 15 rows

B 8 rows

C 5 rows

D 3 rows

💡 Hint

Think about how many pairs you can make by combining each row from A with each row from B.

CROSS JOIN pairs every row from A with every row from B, so total rows = 5 * 3 = 15.

📝 Syntax

advanced

Identify the syntax error in CROSS JOIN query

Which of the following queries will cause a syntax error in PostgreSQL?

A SELECT * FROM table1 CROSS JOIN;

B SELECT * FROM table1, table2;

C SELECT * FROM table1 CROSS JOIN table2 ON table1.id = table2.id;

D SELECT * FROM table1 CROSS JOIN table2;

💡 Hint

Check if the CROSS JOIN clause is complete with both table names.

Option A is missing the second table name after CROSS JOIN, causing a syntax error. Option A is valid syntax but the ON clause is ignored in CROSS JOIN, so no error. Option A uses implicit join syntax and is valid.

❓ optimization

advanced

Performance impact of CROSS JOIN on large tables

You have two large tables: Employees with 10,000 rows and Departments with 100 rows. You run a CROSS JOIN between them without any filtering. What is the main performance concern?

A The query will fail because CROSS JOIN is not allowed on large tables.

B The query will return 10,000 rows, which is easy to handle.

C The query will return 100 rows, so performance impact is minimal.

D The query will return 1,000,000 rows, which can cause high memory and CPU usage.

💡 Hint

Think about how many combinations are created by pairing every row from both tables.

CROSS JOIN returns the Cartesian product, so 10,000 * 100 = 1,000,000 rows. This large result can cause high resource usage and slow performance.

🔧 Debug

expert

Why does this CROSS JOIN query return fewer rows than expected?

You have two tables:

Authors: 4 rows
Books: 5 rows

You run this query:

SELECT * FROM Authors CROSS JOIN Books WHERE Authors.id = Books.author_id;

But the result has only 5 rows. Why?

A The query is invalid because CROSS JOIN cannot have a WHERE clause.

B CROSS JOIN always returns the smaller table's row count, so 5 rows is expected.

C The WHERE clause filters the CROSS JOIN result to only matching author-book pairs, reducing rows.

D The database automatically converts CROSS JOIN to INNER JOIN, so only matching rows appear.

💡 Hint

Consider how WHERE filters affect the Cartesian product result.

The CROSS JOIN creates all combinations (4*5=20 rows), but the WHERE clause keeps only rows where Authors.id matches Books.author_id, resulting in 5 rows.