Learn SQL from fundamentals to advanced interview and
production-level database concepts through practical examples,
questions and quizzes.
Complete SQL Learning & Interview Path
Learn SQL by working with realistic databases instead of
memorising isolated commands. Every concept can be connected
with practical tables such as students, employees,
departments, countries, states, customers, products and orders.
SQL is used to store, retrieve, modify and analyse structured
data. Instead of learning commands independently, imagine that
we are building a small company database.
Student table
A student database can contain identity, name,
course and marks.
CREATE TABLE students (
id INT PRIMARY KEY,
name VARCHAR(100),
course VARCHAR(100),
marks INT
);
Employee table
Employee information can be connected with departments,
salaries and managers.
CREATE TABLE employees (
id INT PRIMARY KEY,
name VARCHAR(100),
department_id INT,
salary DECIMAL(12,2)
);
Country / State table
Geographic data is useful for understanding
relationships and JOINs.
CREATE TABLE states (
id INT PRIMARY KEY,
name VARCHAR(100),
country_id INT
);
Customer / Orders
E-commerce data provides practical examples for
aggregation, JOINs and transactions.
CREATE TABLE orders (
id INT PRIMARY KEY,
customer_id INT,
total DECIMAL(12,2),
order_date DATE
);
id
name
course
marks
1
Rahul
Java
85
2
Priya
SQL
92
3
Amit
React
76
SQL Learning Block 1
Questions
1
–
3
1
Question
21
of 3
Find maximum salary in each department.
Solution
SELECT department_id, MAX(salary) AS max_salary FROM employees GROUP BY department_id;
Think before looking at the solution
What would the SQL query do?
Which rows will match?
Which columns will appear?
What happens when NULL values are involved?
Can the query be improved using an index?
Question
22
of 3
Find departments having more than 5 employees.
Solution
SELECT department_id, COUNT(*) AS employee_count FROM employees GROUP BY department_id HAVING COUNT(*) > 5;
WHERE filters rows; HAVING filters groups.
Think before looking at the solution
What would the SQL query do?
Which rows will match?
Which columns will appear?
What happens when NULL values are involved?
Can the query be improved using an index?
Question
23
of 3
Find departments whose average salary is greater than 60,000.
Solution
SELECT department_id, AVG(salary) AS avg_salary FROM employees GROUP BY department_id HAVING AVG(salary) > 60000;
Think before looking at the solution
What would the SQL query do?
Which rows will match?
Which columns will appear?
What happens when NULL values are involved?
Can the query be improved using an index?
Knowledge Check
Quiz — Final Learning Block
Complete the remaining questions before finishing
this SQL learning path.
1.
Find maximum salary in each department.
Show explanation
SELECT department_id, MAX(salary) AS max_salary FROM employees GROUP BY department_id;
2.
Find departments having more than 5 employees.
Show explanation
SELECT department_id, COUNT(*) AS employee_count FROM employees GROUP BY department_id HAVING COUNT(*) > 5;
WHERE filters rows; HAVING filters groups.
3.
Find departments whose average salary is greater than 60,000.
Show explanation
SELECT department_id, AVG(salary) AS avg_salary FROM employees GROUP BY department_id HAVING AVG(salary) > 60000;
