Natural join and its risks in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

SQL

SELECT * FROM employees [1] departments;

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

ANATURAL JOIN

BINNER JOIN ON

CLEFT JOIN

DCROSS JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN without ON clause causes syntax error.

Using CROSS JOIN returns all combinations, not matching rows.

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

2fill in blank

medium

Complete the code to find all employees with their department names using natural join.

SQL

SELECT employee_name, department_name FROM employees [1] departments;

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

ALEFT JOIN ON employees.department_id = departments.department_id

BNATURAL JOIN

CRIGHT JOIN ON employees.department_id = departments.department_id

DFULL JOIN ON employees.department_id = departments.department_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using LEFT JOIN without ON clause causes error.

Using RIGHT JOIN or FULL JOIN when natural join is intended.

NATURAL JOIN automatically joins on columns with the same name, here department_id.

3fill in blank

hard

Fix the error in the natural join query that causes unexpected results due to extra common columns.

SQL

SELECT * FROM employees [1] departments;

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

ALEFT JOIN ON employees.department_id = departments.department_id

BNATURAL JOIN

CINNER JOIN ON employees.department_id = departments.department_id

DCROSS JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Continuing to use NATURAL JOIN without checking common columns.

Using CROSS JOIN which returns all combinations.

Using INNER JOIN ON with explicit condition avoids risks of natural join matching unintended columns.

4fill in blank

hard

Fill both blanks to create a safe join avoiding natural join risks and selecting employee and department names.

SQL

SELECT employees.employee_name, departments.department_name FROM employees [1] departments [2] employees.department_id = departments.department_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 instead of INNER JOIN.

Using USING keyword which can still cause issues if column names differ.

Using INNER JOIN with ON clause specifies the join condition clearly, avoiding natural join risks.

5fill in blank

hard

Fill all three blanks to write a query that joins employees and departments safely, selecting employee name, department name, and filtering by department location.

SQL

SELECT [1], [2] FROM employees [3] departments ON employees.department_id = departments.department_id WHERE departments.location = 'New York';

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

Aemployees.employee_name

Bdepartments.department_name

CINNER JOIN

DNATURAL JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using NATURAL JOIN which may join on unintended columns.

Selecting columns without table prefixes causing ambiguity.

This query uses INNER JOIN with explicit ON clause to safely join tables and selects the correct columns.

Practice

1. What does a NATURAL JOIN do in SQL?

easy

A. Automatically joins tables on all columns with the same names

B. Joins tables only on columns with different names

C. Joins tables without any condition

D. Joins tables using a specified ON condition

Solution

1. Step 1: Understand the definition of NATURAL JOIN

   A NATURAL JOIN automatically matches columns with the same names in both tables and joins on those columns.

2. Step 2: Compare with other join types

   Unlike explicit ON conditions, NATURAL JOIN uses all common column names without needing to specify them.

3. Final Answer:

   Automatically joins tables on all columns with the same names -> Option A

4. Quick Check:

   NATURAL JOIN = joins on same-named columns [OK]

Hint: Natural join matches all same-named columns automatically [OK]

Common Mistakes:

• Thinking NATURAL JOIN joins on different column names
• Assuming NATURAL JOIN needs ON clause
• Believing NATURAL JOIN joins without any condition

2. Which of the following is the correct syntax for a natural 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 NATURAL JOIN Departments;

D. SELECT * FROM Employees CROSS JOIN Departments NATURAL;

Solution

1. Step 1: Recall the syntax of NATURAL JOIN

   The correct syntax is: SELECT columns FROM table1 NATURAL JOIN table2;

2. Step 2: Check each option

   SELECT * FROM Employees NATURAL JOIN Departments; uses the correct NATURAL JOIN syntax. SELECT * FROM Employees JOIN Departments ON Employees.id = Departments.id; uses explicit ON clause, not natural join. SELECT * FROM Employees INNER JOIN Departments; misses join condition. SELECT * FROM Employees CROSS JOIN Departments NATURAL; is invalid syntax.

3. Final Answer:

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

4. Quick Check:

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

Hint: Natural join syntax: FROM table1 NATURAL JOIN table2 [OK]

Common Mistakes:

• Using ON clause with NATURAL JOIN
• Missing NATURAL keyword
• Placing NATURAL after JOIN keyword incorrectly

3. Given two tables:
Employees(emp_id, name, dept_id)
Departments(dept_id, dept_name, location)
What will be the result of this query?

SELECT emp_id, name, dept_name FROM Employees NATURAL JOIN Departments;

medium

A. Syntax error due to missing ON clause

B. Rows combining employees with their department names based on matching dept_id

C. Only employees with no department

D. All employees repeated for each department

Solution

1. Step 1: Identify common columns for NATURAL JOIN

   Both tables share dept_id, so NATURAL JOIN matches rows where dept_id is equal.

2. Step 2: Understand the output columns

   The query selects emp_id, name from Employees and dept_name from Departments, showing employee info with their department name.

3. Final Answer:

   Rows combining employees with their department names based on matching dept_id -> Option B

4. Quick Check:

   NATURAL JOIN matches on dept_id, returns combined rows [OK]

Hint: Natural join matches on common columns, returns combined rows [OK]

Common Mistakes:

• Expecting all employees repeated for each department
• Thinking NATURAL JOIN returns unmatched rows
• Assuming syntax error without ON clause

4. Consider these tables:
Orders(order_id, customer_id, date)
Customers(customer_id, name, date)
What is the main problem with using NATURAL JOIN on these tables?

medium

A. It will join on both customer_id and date, possibly causing incorrect matches

B. It will cause a syntax error because of duplicate column names

C. It will ignore the customer_id column and join only on date

D. It will return no rows because columns have the same name

Solution

1. Step 1: Identify columns with same names in both tables

   Both tables have customer_id and date columns.

2. Step 2: Understand NATURAL JOIN behavior

   NATURAL JOIN joins on all columns with the same names, so it will join on both customer_id and date, which may cause unintended filtering or incorrect matches.

3. Final Answer:

   It will join on both customer_id and date, possibly causing incorrect matches -> Option A

4. Quick Check:

   NATURAL JOIN joins on all same-named columns, beware unintended matches [OK]

Hint: Natural join joins on all same-named columns, watch for unintended matches [OK]

Common Mistakes:

• Thinking NATURAL JOIN causes syntax errors with duplicate columns
• Assuming it joins only on one column
• Believing it returns no rows due to same column names

5. You have two tables:
Products(product_id, name, category_id)
Categories(category_id, name)
Using NATURAL JOIN between these tables causes unexpected results. What is the best way to fix this?

hard

A. Remove the category_id column from one table

B. Use NATURAL JOIN anyway and ignore the extra matches

C. Use CROSS JOIN to avoid matching columns

D. Rename one of the name columns and use an explicit JOIN with ON clause

Solution

1. Step 1: Identify the cause of unexpected results

   Both tables have a column named name. NATURAL JOIN joins on all same-named columns, so it joins on category_id and name, causing unintended matches.

2. Step 2: Fix by renaming and using explicit join

   Renaming one name column (e.g., to category_name) and using an explicit JOIN with ON clause on category_id avoids accidental joins on name.

3. Final Answer:

   Rename one of the name columns and use an explicit JOIN with ON clause -> Option D

4. Quick Check:

   Rename columns + explicit ON join avoids NATURAL JOIN risks [OK]

Hint: Rename columns and use explicit ON join to avoid natural join risks [OK]

Common Mistakes:

• Ignoring column name conflicts with NATURAL JOIN
• Removing important columns instead of renaming
• Using CROSS JOIN which returns all combinations