Basic · Lesson 1 of 14

What is SQL

The one language every data job interviews around.

A language for asking questions of tables

SQL is how you talk to relational databases. A database is a collection of tables, each table is a grid of rows and columns, and SQL lets you filter, combine, and summarize those rows without loading them into Excel. Almost every technical data interview opens with a SQL question, so fluency here compounds into everything else.

The core shape

Nearly every query you will ever write fits a small template. You select columns, you say which table they come from, you optionally filter rows, and you optionally sort. Interviews rarely test exotic syntax. They test whether you can use this template to answer messy business questions.

Why this matters in 2026

AI assistants now draft SQL fast, which raised the bar. Interviews ask you to review generated queries, spot the wrong join, and explain a metric precisely. Syntax you can look up; the ability to reason about what a query returns is what gets graded.

Examples

Read the whole table

The star means "every column". Fine for exploring, but list columns explicitly in real work.

SELECT * FROM users;

List just the names and plans

Choosing columns first is a habit that keeps queries readable.

SELECT name, plan FROM users;