Self join concept in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all employees and their managers using a self join.

SQL

SELECT e.name AS employee, m.name AS manager FROM employees e JOIN employees m ON e.manager_id = [1].id;

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

Aemployees

Bm

Ce

Dmanagers

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same alias for both tables causes confusion.

Joining on the wrong columns or missing the alias before the column.

The alias m refers to the managers table (which is the same employees table but aliased). We join on e.manager_id = m.id to link employees to their managers.

2fill in blank

medium

Complete the code to find employees whose manager's name is 'Alice'.

SQL

SELECT e.name FROM employees e JOIN employees m ON e.manager_id = m.id WHERE m.name = [1];

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

A'Bob'

B'Manager'

C'Alice'

D'John'

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the string value.

Using the employee's name instead of the manager's name in the WHERE clause.

We want to find employees whose manager's name is 'Alice'. So the condition is m.name = 'Alice'.

3fill in blank

hard

Fix the error in the code to correctly list employees and their managers.

SQL

SELECT e.name, m.name FROM employees e JOIN employees m ON e.manager_id = [1].id;

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

Am

Be

Cemployees

Dmanagers

Attempts:

3 left

💡 Hint

Common Mistakes

Joining on the wrong column causing no results or errors.

Using the employee alias instead of the manager alias in the join condition.

The join condition should match e.manager_id = m.id, not m.manager_id. So the alias before id is m.

4fill in blank

hard

Fill both blanks to select employees and their managers who work in the same department.

SQL

SELECT e.name, m.name FROM employees e JOIN employees m ON e.manager_id = m.id WHERE e.[1] = m.[2];

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

Adepartment_id

Bmanager_id

Cid

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using manager_id or id instead of department_id for the comparison.

Comparing employee's department_id with manager's name or other unrelated columns.

To find employees and managers in the same department, compare e.department_id with m.department_id.

5fill in blank

hard

Fill all three blanks to create a query that lists employees, their managers, and the manager's department name.

SQL

SELECT e.name AS employee, m.name AS manager, d.[1] FROM employees e JOIN employees m ON e.[2] = m.[3] JOIN departments d ON m.department_id = d.id;

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

Adepartment_name

Bmanager_id

Cid

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong column names for joins causing errors.

Selecting wrong columns from the departments table.

The query selects the department name from the departments table. The join between employees and managers is on e.manager_id = m.id.

Practice

1. What is the main purpose of a self join in SQL?

easy

A. To combine rows from two tables without any condition

B. To join two different tables based on a common column

C. To join a table to itself to compare rows within the same table

D. To delete duplicate rows from a table

Solution

1. Step 1: Understand the concept of self join

   A self join is used when you want to compare rows within the same table by treating it as two separate tables using aliases.

2. Step 2: Identify the correct purpose

   Joining a table to itself allows you to find relationships or comparisons between rows in the same table, such as hierarchical data or pairs.

3. Final Answer:

   To join a table to itself to compare rows within the same table -> Option C

4. Quick Check:

   Self join = join table to itself [OK]

Hint: Self join means joining table to itself using aliases [OK]

Common Mistakes:

• Confusing self join with joining two different tables
• Thinking self join deletes duplicates
• Assuming self join combines rows without condition

2. Which of the following is the correct syntax to perform a self join on a table named employees with alias e1 and e2?

easy

A. SELECT * FROM employees e1 JOIN employees e2 ON e1.id = e2.manager_id;

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

C. SELECT * FROM employees e1, employees e2 ON e1.id = e2.manager_id;

D. SELECT * FROM employees e1 INNER JOIN employees e2 ON e1.id = e2.id;

Solution

1. Step 1: Use table aliases for self join

   To join a table to itself, you must use aliases like e1 and e2 to distinguish the two instances.

2. Step 2: Write the join condition correctly

   The join condition should relate columns from the two aliases, for example e1.id = e2.manager_id to find employees and their managers.

3. Final Answer:

   SELECT * FROM employees e1 JOIN employees e2 ON e1.id = e2.manager_id; -> Option A

4. Quick Check:

   Self join syntax = table alias + join condition [OK]

Hint: Always use aliases to distinguish the same table twice [OK]

Common Mistakes:

• Not using aliases causes syntax errors
• Joining on wrong columns returns wrong results
• Using comma join without aliases is confusing

3. Given the table employees with columns id, name, and manager_id, what will this query return?

SELECT e1.name AS Employee, e2.name AS Manager FROM employees e1 LEFT JOIN employees e2 ON e1.manager_id = e2.id;

medium

A. Syntax error due to missing WHERE clause

B. List of employees with their managers' names, NULL if no manager

C. List of employees without managers only

D. List of managers with their employees' names

Solution

1. Step 1: Understand the LEFT JOIN on self join

   The query joins the employees table to itself using aliases e1 and e2, matching e1.manager_id to e2.id to find each employee's manager.

2. Step 2: Interpret the SELECT columns and join type

   Using LEFT JOIN means all employees (e1) appear, even if they have no manager (e2.name will be NULL). The SELECT shows employee and manager names.

3. Final Answer:

   List of employees with their managers' names, NULL if no manager -> Option B

4. Quick Check:

   LEFT JOIN self join shows all employees with managers [OK]

Hint: LEFT JOIN keeps all employees, shows NULL for missing managers [OK]

Common Mistakes:

• Confusing employee and manager columns
• Thinking it lists only managers or only employees without managers
• Assuming syntax error without WHERE clause

4. Identify the error in this self join query:

SELECT e1.name, e2.name FROM employees e1 JOIN employees e2 ON e1.id = e2.id;

medium

A. Missing table aliases for employees

B. Using JOIN instead of LEFT JOIN causes error

C. Syntax error due to missing WHERE clause

D. The join condition compares the same column, causing incorrect results

Solution

1. Step 1: Analyze the join condition

   The query joins employees to itself on e1.id = e2.id, which matches each row to itself only, not to related rows.

2. Step 2: Understand the effect of the condition

   This join condition does not find relationships like manager or pairs; it just duplicates rows, which is likely incorrect for self join use.

3. Final Answer:

   The join condition compares the same column, causing incorrect results -> Option D

4. Quick Check:

   Self join needs meaningful join condition, not same column equals [OK]

Hint: Join condition must relate different columns for meaningful self join [OK]

Common Mistakes:

• Joining on identical columns returns only same rows
• Forgetting to use aliases
• Assuming JOIN requires WHERE clause

5. You have a table employees with columns id, name, and manager_id. Write a query using self join to find all employees who share the same manager. Which query correctly achieves this?

hard

A. SELECT e1.name, e2.name FROM employees e1 JOIN employees e2 ON e1.manager_id = e2.manager_id WHERE e1.id <> e2.id;

B. SELECT e1.name, e2.name FROM employees e1 JOIN employees e2 ON e1.id = e2.manager_id;

C. SELECT e1.name, e2.name FROM employees e1 JOIN employees e2 ON e1.manager_id = e2.id WHERE e1.id = e2.id;

D. SELECT e1.name, e2.name FROM employees e1 JOIN employees e2 ON e1.manager_id = e2.manager_id WHERE e1.id = e2.id;

Solution

1. Step 1: Understand the goal

   We want pairs of employees who share the same manager, so their manager_id values must be equal but employees must be different.

2. Step 2: Write the self join condition

   Joining on e1.manager_id = e2.manager_id finds employees with the same manager. Adding WHERE e1.id <> e2.id excludes pairing an employee with themselves.

3. Final Answer:

   SELECT e1.name, e2.name FROM employees e1 JOIN employees e2 ON e1.manager_id = e2.manager_id WHERE e1.id <> e2.id; -> Option A

4. Quick Check:

   Same manager and different employees = SELECT e1.name, e2.name FROM employees e1 JOIN employees e2 ON e1.manager_id = e2.manager_id WHERE e1.id <> e2.id; [OK]

Hint: Join on manager_id and exclude same employee IDs [OK]

Common Mistakes:

• Joining on employee id instead of manager id
• Not excluding same employee pairs
• Using equality on employee IDs causing no pairs