NATURAL join and its risks in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to perform a natural join between tables employees and departments.

PostgreSQL

SELECT * FROM employees [1] departments;

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

AINNER JOIN ON

BCROSS JOIN

CLEFT JOIN

DNATURAL JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN without ON clause causes syntax error.

Using CROSS JOIN returns Cartesian product, not matching rows.

The NATURAL JOIN keyword joins two tables based on all columns with the same names in both tables.

2fill in blank

medium

Complete the code to select employee names and their department names using NATURAL JOIN.

PostgreSQL

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

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

ANATURAL JOIN

BJOIN ON employees.dept_id = departments.id

CLEFT JOIN ON employees.dept_id = departments.id

DCROSS JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using JOIN ON without specifying the join condition.

Using CROSS JOIN which returns all combinations.

NATURAL JOIN automatically joins on columns with the same name, so no ON clause is needed.

3fill in blank

hard

Fix the error in the query that uses NATURAL JOIN but returns unexpected results due to extra matching columns.

PostgreSQL

SELECT * FROM orders [1] customers ON orders.customer_id = customers.id;

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

ANATURAL JOIN

BINNER JOIN

CLEFT JOIN

DCROSS JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using NATURAL JOIN when tables have multiple columns with the same name but different meanings.

Using CROSS JOIN which returns all rows combined.

Using INNER JOIN ON with explicit join condition avoids unexpected matches caused by NATURAL JOIN on multiple columns.

4fill in blank

hard

Fill both blanks to create a query that avoids NATURAL JOIN risks by specifying join columns explicitly.

PostgreSQL

SELECT * FROM sales [1] customers [2] sales.customer_id = customers.id;

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

AINNER JOIN

BLEFT JOIN

CON

DUSING

Attempts:

3 left

💡 Hint

Common Mistakes

Using NATURAL JOIN which can join on unintended columns.

Using USING keyword without matching column names.

Using INNER JOIN with ON clause explicitly defines the join condition, avoiding risks of NATURAL JOIN.

5fill in blank

hard

Fill all three blanks to write a safe join query selecting employee and department names, avoiding NATURAL JOIN risks.

PostgreSQL

SELECT e.name AS employee_name, d.name AS department_name FROM employees e [1] departments d [2] e.dept_id [3] d.id;

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

AINNER JOIN

B=

CON

DLEFT JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using NATURAL JOIN which may join on unintended columns.

Using INNER JOIN when you want to keep all employees regardless of department.

Using LEFT JOIN with ON clause and explicit equality condition safely joins tables without NATURAL JOIN risks.

Practice

1. What does a NATURAL JOIN do in PostgreSQL?

easy

A. It joins tables automatically on all columns with the same names.

B. It joins tables only on the primary key columns.

C. It joins tables using a user-specified condition.

D. It joins tables without any condition, creating a Cartesian product.

Solution

1. Step 1: Understand NATURAL JOIN behavior

   NATURAL JOIN matches columns with the same names in both tables automatically.

2. Step 2: Compare with other join types

   Unlike explicit JOIN ON, NATURAL JOIN does not require specifying join conditions; it uses all common column names.

3. Final Answer:

   It joins tables automatically on all columns with the same names. -> Option A

4. Quick Check:

   NATURAL JOIN = automatic matching columns [OK]

Hint: NATURAL JOIN matches all same-named columns automatically [OK]

Common Mistakes:

• Thinking NATURAL JOIN uses only primary keys
• Confusing NATURAL JOIN with CROSS JOIN
• Assuming NATURAL JOIN requires explicit ON condition

2. Which of the following is the correct syntax for a NATURAL JOIN between tables employees and departments?

easy

A. SELECT * FROM employees NATURAL JOIN departments;

B. SELECT * FROM employees JOIN departments ON employees.id = departments.id;

C. SELECT * FROM employees CROSS JOIN departments;

D. SELECT * FROM employees INNER JOIN departments USING (id);

Solution

1. Step 1: Identify NATURAL JOIN syntax

   The correct syntax uses the keyword NATURAL JOIN directly between table names.

2. Step 2: Compare options

   SELECT * FROM employees NATURAL JOIN departments; uses NATURAL JOIN correctly; others use different join types or explicit conditions.

3. Final Answer:

   SELECT * FROM employees NATURAL JOIN departments; -> Option A

4. Quick Check:

   Correct NATURAL JOIN syntax = SELECT * FROM employees NATURAL JOIN departments; [OK]

Hint: NATURAL JOIN keyword goes directly between table names [OK]

Common Mistakes:

• Using ON clause with NATURAL JOIN
• Confusing NATURAL JOIN with USING clause
• Writing CROSS JOIN instead of NATURAL JOIN

3. Given two tables:
students(id, name, class)
enrollments(id, class, grade)
What will be the result of this query?

SELECT * FROM students NATURAL JOIN enrollments;

medium

A. All rows from students combined with all rows from enrollments.

B. Rows where both id and class match in both tables.

C. Rows where only class matches in both tables.

D. Rows where only id matches in both tables.

Solution

1. Step 1: Identify common columns

   Both tables share columns id and class.

2. Step 2: Understand NATURAL JOIN behavior

   NATURAL JOIN matches on all columns with the same names, so it joins on both id and class.

3. Final Answer:

   Rows where both id and class match in both tables. -> Option B

4. Quick Check:

   NATURAL JOIN matches all common columns [OK]

Hint: NATURAL JOIN uses all same-named columns to join [OK]

Common Mistakes:

• Assuming join on only one common column
• Confusing NATURAL JOIN with JOIN USING
• Expecting Cartesian product instead of filtered rows

4. Consider these tables:
products(id, name, price)
sales(id, product_id, quantity)
What is the problem with this query?

SELECT * FROM products NATURAL JOIN sales;

medium

A. It will return all rows without filtering.

B. It will cause a syntax error because NATURAL JOIN requires USING clause.

C. It will join on id columns, which are different keys, causing incorrect results.

D. It will join correctly on product_id automatically.

Solution

1. Step 1: Identify common columns

   Both tables have a column named id, but they represent different things.

2. Step 2: Understand NATURAL JOIN risk

   NATURAL JOIN joins on all same-named columns, so it joins on id which mismatches keys, causing wrong results.

3. Final Answer:

   It will join on id columns, which are different keys, causing incorrect results. -> Option C

4. Quick Check:

   Unintended column match = wrong join [OK]

Hint: Check if same-named columns mean same data before NATURAL JOIN [OK]

Common Mistakes:

• Assuming NATURAL JOIN always joins correctly
• Expecting NATURAL JOIN to use foreign keys automatically
• Ignoring column name conflicts causing wrong joins

5. You have two tables:
orders(order_id, customer_id, date)
customers(customer_id, name, city)
You want to join them but avoid errors from unexpected columns with the same name. Which is the safest approach?

hard

A. Use NATURAL JOIN only if you are sure about column names and their meaning.

B. Always use NATURAL JOIN for simplicity, ignoring column details.

C. Use CROSS JOIN and filter results manually.

D. Use INNER JOIN with explicit ON condition specifying join columns.

Solution

1. Step 1: Understand risks of NATURAL JOIN

   NATURAL JOIN can cause errors if tables have unintended same-named columns.

2. Step 2: Choose safer join method

   INNER JOIN with explicit ON condition avoids ambiguity by specifying exact join columns.

3. Final Answer:

   Use INNER JOIN with explicit ON condition specifying join columns. -> Option D

4. Quick Check:

   Explicit ON join = safest to avoid NATURAL JOIN risks [OK]

Hint: Use explicit ON join to avoid NATURAL JOIN surprises [OK]

Common Mistakes:

• Blindly using NATURAL JOIN without checking columns
• Using CROSS JOIN expecting filtered results
• Ignoring join conditions leads to wrong data