Overview

SQL (Structured Query Language) is the standard language for working with relational databases such as PostgreSQL, MySQL, Oracle and SQL Server. Almost every application stores data in a database, so SQL is one of the most valuable skills you can have.

We start with the basics and move to the topics that matter in real projects and interviews: joins, grouping, subqueries, indexes and transactions.

Where it is used

• Reports and dashboards
• Application back-ends
• Data analysis
• Data migration and clean-up

Example queries

SELECT d.name AS department, COUNT(e.id) AS employees, AVG(e.salary) AS avg_salary
FROM department d
LEFT JOIN employee e ON e.department_id = d.id
GROUP BY d.name
HAVING COUNT(e.id) > 0
ORDER BY avg_salary DESC;