Skill guide

SQL

SQL is used in nearly every technical role that touches data: analysts, backend engineers, and data scientists are all expected to write it well.

Skill levels

beginner

  • SELECT, WHERE, ORDER BY
  • Basic joins (INNER, LEFT)
  • GROUP BY and aggregate functions

intermediate

  • Subqueries and CTEs
  • Window functions
  • Schema design and normalization basics

Job-ready

  • Optimize a slow query using indexes/EXPLAIN
  • Design a schema for a real multi-table app
  • Write complex analytical queries under time pressure

How to prove it

  • A completed assessment
  • A schema + query set from a real project
  • Live-solved query exercises with explanation

Common interview questions

Mentor opinion
  • Write a query to find the second-highest value in a column.
  • Explain the difference between WHERE and HAVING.
  • How would you find duplicate rows in a table?