Joins in SQL in DBMS Theory - Mini Project: Build & Apply

Joins in SQL

📖 Scenario: You are working with two tables in a company's database: employees and departments. You want to learn how to combine data from these tables to see which employee works in which department.

🎯 Goal: Build SQL queries step-by-step to join the employees and departments tables using different types of joins.

📋 What You'll Learn

Create the employees table with exact columns and data

Create the departments table with exact columns and data

Write a SQL query using INNER JOIN to combine employees with their departments

Write a SQL query using LEFT JOIN to include all employees even if they have no department

💡 Why This Matters

🌍 Real World

Joining tables is a common task in databases to combine information like employee details with their department names.

💼 Career

Understanding SQL joins is essential for data analysts, database administrators, and developers to query and report data effectively.

1

Create the employees table with data

Write SQL statements to create a table called employees with columns id (integer), name (text), and department_id (integer). Insert these exact rows: (1, 'Alice', 10), (2, 'Bob', 20), (3, 'Charlie', NULL).

DBMS Theory

-- Create the employees table and insert data
-- Your code here

Hint

Use CREATE TABLE to define the table and INSERT INTO to add rows.

2

Create the departments table with data

Write SQL statements to create a table called departments with columns id (integer) and name (text). Insert these exact rows: (10, 'HR'), (20, 'Engineering'), (30, 'Marketing').

DBMS Theory

CREATE TABLE employees (
  id INTEGER,
  name TEXT,
  department_id INTEGER
);

INSERT INTO employees (id, name, department_id) VALUES
  (1, 'Alice', 10),
  (2, 'Bob', 20),
  (3, 'Charlie', NULL);

-- Create the departments table and insert data
-- Your code here

Hint

Use the same pattern as the employees table to create departments.

3

Write an INNER JOIN query to combine employees with their departments

Write a SQL query that selects employees.name and departments.name as department using an INNER JOIN on employees.department_id = departments.id.

DBMS Theory

CREATE TABLE employees (
  id INTEGER,
  name TEXT,
  department_id INTEGER
);

INSERT INTO employees (id, name, department_id) VALUES
  (1, 'Alice', 10),
  (2, 'Bob', 20),
  (3, 'Charlie', NULL);

CREATE TABLE departments (
  id INTEGER,
  name TEXT
);

INSERT INTO departments (id, name) VALUES
  (10, 'HR'),
  (20, 'Engineering'),
  (30, 'Marketing');

-- Write INNER JOIN query below
-- Your code here

Hint

Use INNER JOIN to combine rows where the department matches.

4

Write a LEFT JOIN query to include all employees even without a department

Write a SQL query that selects employees.name and departments.name as department using a LEFT JOIN on employees.department_id = departments.id to include employees without a department.

DBMS Theory

CREATE TABLE employees (
  id INTEGER,
  name TEXT,
  department_id INTEGER
);

INSERT INTO employees (id, name, department_id) VALUES
  (1, 'Alice', 10),
  (2, 'Bob', 20),
  (3, 'Charlie', NULL);

CREATE TABLE departments (
  id INTEGER,
  name TEXT
);

INSERT INTO departments (id, name) VALUES
  (10, 'HR'),
  (20, 'Engineering'),
  (30, 'Marketing');

SELECT employees.name, departments.name AS department
FROM employees
INNER JOIN departments ON employees.department_id = departments.id;

-- Write LEFT JOIN query below
-- Your code here

Hint

Use LEFT JOIN to include all employees, even those without a matching department.

Practice

1. What does an INNER JOIN do in SQL?

easy

A. Returns all rows from the right table, and matched rows from the left

B. Returns all rows from the left table, and matched rows from the right

C. Returns only rows with matching values in both tables

D. Returns all rows from both tables, matching where possible

Solution

1. Step 1: Understand INNER JOIN behavior

   An INNER JOIN returns rows where there is a match in both tables based on the join condition.

2. Step 2: Compare with other join types

   LEFT JOIN returns all from left, RIGHT JOIN all from right, FULL JOIN all from both. INNER JOIN only matched rows.

3. Final Answer:

   Returns only rows with matching values in both tables -> Option C

4. Quick Check:

   INNER JOIN = matched rows only [OK]

Hint: INNER JOIN keeps only matching rows from both tables [OK]

Common Mistakes:

• Confusing INNER JOIN with LEFT JOIN
• Thinking INNER JOIN returns unmatched rows
• Mixing up LEFT and RIGHT JOIN definitions

2. Which of the following is the correct syntax for a LEFT JOIN in SQL?

easy

A. SELECT * FROM table1 LEFT JOIN table2 USING (id);

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

C. SELECT * FROM table1 LEFT OUTER JOIN table2 WHERE table1.id = table2.id;

D. SELECT * FROM table1 LEFT JOIN table2 ON table1.id = table2.id;

Solution

1. Step 1: Recall LEFT JOIN syntax

   The correct syntax is: SELECT columns FROM table1 LEFT JOIN table2 ON condition;

2. Step 2: Check each option

   SELECT * FROM table1 LEFT JOIN table2 ON table1.id = table2.id; matches correct syntax. SELECT * FROM table1 JOIN LEFT table2 ON table1.id = table2.id; has wrong order. SELECT * FROM table1 LEFT OUTER JOIN table2 WHERE table1.id = table2.id; uses WHERE instead of ON. SELECT * FROM table1 LEFT JOIN table2 USING (id); uses correct syntax with parentheses.

3. Final Answer:

   SELECT * FROM table1 LEFT JOIN table2 ON table1.id = table2.id; -> Option D

4. Quick Check:

   LEFT JOIN syntax = FROM table1 LEFT JOIN table2 ON condition [OK]

Hint: LEFT JOIN uses ON, not WHERE, for join condition [OK]

Common Mistakes:

• Using WHERE instead of ON for join condition
• Incorrect order of JOIN keywords
• Confusing USING with ON without matching columns

3. Given two tables:
Employees(emp_id, name, dept_id)
Departments(dept_id, dept_name)
What will this query return?

SELECT e.name, d.dept_name FROM Employees e INNER JOIN Departments d ON e.dept_id = d.dept_id;

medium

A. All employees with their department names, only if dept_id matches

B. All employees, even if they have no department

C. All departments with employees, including departments with no employees

D. All employees and departments, matched or not

Solution

1. Step 1: Analyze INNER JOIN condition

   The INNER JOIN returns rows where Employees.dept_id matches Departments.dept_id.

2. Step 2: Understand result rows

   Only employees with a matching department will appear, showing employee name and department name.

3. Final Answer:

   All employees with their department names, only if dept_id matches -> Option A

4. Quick Check:

   INNER JOIN filters to matching dept_id rows [OK]

Hint: INNER JOIN shows only matching rows from both tables [OK]

Common Mistakes:

• Assuming employees without departments appear
• Confusing INNER JOIN with LEFT JOIN
• Thinking unmatched departments appear

4. Identify the error in this SQL join query:

SELECT * FROM Orders o LEFT JOIN Customers c ON o.customer_id = c.id WHERE c.country = 'USA';

medium

A. The WHERE clause filters out unmatched rows, defeating LEFT JOIN purpose

B. The ON clause should use = instead of LIKE

C. LEFT JOIN should be RIGHT JOIN here

D. Missing GROUP BY clause for aggregation

Solution

1. Step 1: Understand LEFT JOIN with WHERE filter

   LEFT JOIN keeps all rows from Orders, matching Customers or NULL if no match.

2. Step 2: Effect of WHERE on joined table

   WHERE c.country = 'USA' removes rows where c is NULL (no match), so unmatched rows are excluded, negating LEFT JOIN effect.

3. Final Answer:

   The WHERE clause filters out unmatched rows, defeating LEFT JOIN purpose -> Option A

4. Quick Check:

   WHERE on joined table filters NULLs, breaks LEFT JOIN [OK]

Hint: Use ON for join filters, WHERE filters after join [OK]

Common Mistakes:

• Filtering joined table in WHERE instead of ON
• Confusing LEFT JOIN with RIGHT JOIN
• Adding unnecessary GROUP BY

5. You want to list all products and their suppliers, including products without suppliers and suppliers without products. Which join should you use?

hard

A. LEFT JOIN

B. FULL OUTER JOIN

C. RIGHT JOIN

D. INNER JOIN

Solution

1. Step 1: Understand join requirements

   We want all products and all suppliers, even if no matching row exists in the other table.

2. Step 2: Match join type to requirement

   INNER JOIN excludes unmatched rows. LEFT or RIGHT JOIN include unmatched rows from one side only. FULL OUTER JOIN includes unmatched rows from both tables.

3. Final Answer:

   FULL OUTER JOIN -> Option B

4. Quick Check:

   FULL OUTER JOIN returns all rows from both tables [OK]

Hint: Use FULL OUTER JOIN to include unmatched rows from both tables [OK]

Common Mistakes:

• Choosing INNER JOIN which excludes unmatched rows
• Using LEFT or RIGHT JOIN which include only one side unmatched
• Confusing FULL OUTER JOIN syntax