CROSS JOIN behavior in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all columns from both tables using CROSS JOIN.

PostgreSQL

SELECT * FROM table1 [1] table2;

Drag options to blanks, or click blank then click option'

ACROSS JOIN

BINNER JOIN

CLEFT JOIN

DRIGHT JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN instead of CROSS JOIN.

Forgetting that CROSS JOIN does not need an ON clause.

The CROSS JOIN returns the Cartesian product of rows from both tables.

2fill in blank

medium

Complete the code to count the total number of rows after a CROSS JOIN between two tables.

PostgreSQL

SELECT COUNT(*) FROM tableA [1] tableB;

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AFULL JOIN

BINNER JOIN

CLEFT JOIN

DCROSS JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN which filters rows.

Using FULL JOIN which includes NULLs.

CROSS JOIN returns all combinations of rows, so counting rows after CROSS JOIN gives the product of row counts.

3fill in blank

hard

Fix the error in the query to correctly perform a CROSS JOIN between employees and departments.

PostgreSQL

SELECT employees.name, departments.name FROM employees [1] departments;

Drag options to blanks, or click blank then click option'

AINNER JOIN

BCROSS JOIN

CLEFT JOIN

DRIGHT JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using ON clause with CROSS JOIN.

Confusing CROSS JOIN with INNER JOIN.

CROSS JOIN does not use an ON clause. Remove the ON condition to fix the error.

4fill in blank

hard

Fill both blanks to create a query that selects all pairs of product and color using CROSS JOIN and orders by product name.

PostgreSQL

SELECT products.name, colors.name FROM products [1] colors ORDER BY products.[2];

Drag options to blanks, or click blank then click option'

ACROSS JOIN

BINNER JOIN

Cname

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN instead of CROSS JOIN.

Ordering by a wrong column.

Use CROSS JOIN to combine all products and colors. Order by the product name.

5fill in blank

hard

Fill all three blanks to create a query that selects employee names, department names, and their combined count using CROSS JOIN and GROUP BY.

PostgreSQL

SELECT employees.[1], departments.[2], COUNT(*) FROM employees [3] departments GROUP BY employees.name, departments.name;

Drag options to blanks, or click blank then click option'

Aname

Bdepartment_name

CCROSS JOIN

DINNER JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN instead of CROSS JOIN.

Selecting wrong column names.

Select employee name and department department_name. Use CROSS JOIN to combine all pairs and count them.

Practice

1. What does a CROSS JOIN do in PostgreSQL?

easy

A. Pairs every row from the first table with every row from the second table.

B. Returns only matching rows based on a condition.

C. Joins tables based on a common column with duplicates removed.

D. Filters rows from the first table that exist in the second table.

Solution

1. Step 1: Understand CROSS JOIN concept

   A CROSS JOIN creates combinations of all rows from both tables without any condition.

2. Step 2: Compare with other JOIN types

   Unlike INNER JOIN or LEFT JOIN, CROSS JOIN does not filter or match rows but pairs all possible combinations.

3. Final Answer:

   Pairs every row from the first table with every row from the second table. -> Option A

4. Quick Check:

   CROSS JOIN = all combinations [OK]

Hint: Remember: CROSS JOIN multiplies rows from both tables [OK]

Common Mistakes:

• Confusing CROSS JOIN with INNER JOIN
• Thinking CROSS JOIN filters rows
• Assuming CROSS JOIN requires a join condition

2. Which of the following is the correct syntax for a CROSS JOIN in PostgreSQL?

easy

A. SELECT * FROM table1 CROSS JOIN table2;

B. SELECT * FROM table1 JOIN table2 ON table1.id = table2.id;

C. SELECT * FROM table1 INNER CROSS JOIN table2;

D. SELECT * FROM table1 CROSS JOIN ON table2;

Solution

1. Step 1: Recall CROSS JOIN syntax

   The correct syntax is SELECT * FROM table1 CROSS JOIN table2; without any ON clause.

2. Step 2: Identify incorrect options

   SELECT * FROM table1 JOIN table2 ON table1.id = table2.id; uses INNER JOIN with ON clause, C has invalid keyword INNER CROSS JOIN, A has misplaced ON keyword.

3. Final Answer:

   SELECT * FROM table1 CROSS JOIN table2; -> Option A

4. Quick Check:

   Syntax for CROSS JOIN = SELECT * FROM table1 CROSS JOIN table2; [OK]

Hint: CROSS JOIN syntax has no ON clause [OK]

Common Mistakes:

• Adding ON clause with CROSS JOIN
• Using INNER CROSS JOIN which is invalid
• Misplacing keywords in the query

3. You have two tables:
Colors:
color_id | color_name
1 | Red
2 | Blue

Shapes:
shape_id | shape_name
1 | Circle
2 | Square
3 | Triangle

How many rows will this query return?
SELECT color_name, shape_name FROM Colors CROSS JOIN Shapes;

easy

A. 5

B. 3

C. 2

D. 6

Solution

1. Step 1: Count rows in each table

   Colors has 2 rows, Shapes has 3 rows.

2. Step 2: Calculate CROSS JOIN result size

   Result rows = 2 x 3 = 6 rows, all combinations of colors and shapes.

3. Final Answer:

   6 -> Option D

4. Quick Check:

   2 x 3 = 6 rows [OK]

Hint: Multiply row counts of both tables for CROSS JOIN size [OK]

Common Mistakes:

• Adding rows instead of multiplying
• Ignoring one table's row count
• Confusing CROSS JOIN with INNER JOIN

4. Given two tables:
Table A:
id | name
1 | Alice
2 | Bob

Table B:
code
X
Y

What is the result of:
SELECT A.name, B.code FROM A CROSS JOIN B;

medium

A. [('Alice', 'X'), ('Bob', 'Y')]

B. [('Alice', 'X'), ('Alice', 'Y'), ('Bob', 'X'), ('Bob', 'Y')]

C. [('Alice', 'X'), ('Bob', 'X'), ('Alice', 'Y'), ('Bob', 'Y')]

D. [('Alice', 'X'), ('Bob', 'X')]

Solution

1. Step 1: Understand CROSS JOIN output

   Each row in A pairs with every row in B, so 2 rows in A x 2 rows in B = 4 rows total.

2. Step 2: List all combinations

   Pairs are (Alice, X), (Alice, Y), (Bob, X), (Bob, Y) in that order.

3. Final Answer:

   [('Alice', 'X'), ('Alice', 'Y'), ('Bob', 'X'), ('Bob', 'Y')] -> Option B

4. Quick Check:

   2 x 2 = 4 rows [OK]

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

Common Mistakes:

• Assuming CROSS JOIN filters rows
• Mixing order of pairs incorrectly
• Counting only matching rows instead of all combinations

5. Consider this query:
SELECT * FROM table1 CROSS JOIN table2 ON table1.id = table2.id;
What is the problem with this query?

medium

A. It behaves like INNER JOIN with ON condition.

B. It will return only matching rows based on id.

C. Syntax error because CROSS JOIN does not allow ON clause.

D. It returns all rows from table1 only.

Solution

1. Step 1: Check CROSS JOIN syntax rules

   CROSS JOIN syntax does not support ON clause; adding ON causes syntax error.

2. Step 2: Understand error cause

   ON clause is used with INNER or LEFT JOIN, not CROSS JOIN.

3. Final Answer:

   Syntax error because CROSS JOIN does not allow ON clause. -> Option C

4. Quick Check:

   CROSS JOIN + ON = syntax error [OK]

Hint: No ON clause allowed with CROSS JOIN [OK]

Common Mistakes:

• Thinking CROSS JOIN can have ON clause
• Confusing CROSS JOIN with INNER JOIN
• Expecting filtered results from CROSS JOIN