SQL

The language every backend job assumes you already know, from SELECT and joins through NULL logic, window functions and constraints to transactions and indexes.

Start the course Practise this with real questions

In this course

SELECTAsking a table for some of its columns, and nothing else.5 min
WHEREKeeping the rows you want, and the one comparison that never works.6 min
ORDER BY and LIMITPutting rows in an order, and why they have none until you say so.5 min
GROUP BY and aggregatesCollapsing many rows into one number, and the difference between WHERE and HAVING.7 min
JoinsPutting two tables together, and the WHERE clause that silently undoes a LEFT JOIN.8 min
NULL and three-valued logicWhy SQL has three truth values, where NULL leaks into answers you thought were safe, and how to compare columns that may be missing.9 min
Types, CAST and CASEHow a column's type decides the arithmetic, what CAST does and refuses, and CASE as the tool for conditional counts.10 min
Subqueries, EXISTS and derived tablesA query inside a query — as a single value, as a test, or as a table — and the NOT IN that returns nothing.10 min
UNION, INTERSECT and EXCEPTCombining whole result sets — the duplicate removal UNION does for you, the columns that line up by position, and NULLs that suddenly match.8 min
Common table expressionsNaming a subquery with WITH so a query reads top to bottom, and the recursive form that walks a hierarchy.10 min
Window functionsAggregates that keep every row — ranking, running totals and comparing a row with its neighbour — and the default frame that surprises people.12 min
INSERT, UPDATE and DELETEChanging rows safely — inserting from a query, updating from another table without wiping the rows that had no match, and deleting with EXISTS.10 min
Constraints and keysPrimary, unique and foreign keys, NOT NULL and CHECK — what each one refuses, the NULLs each one lets through, and what happens on delete.10 min
NormalizationStoring each fact once — the anomalies a flat table invites, the normal forms that rule them out, and when to break the rules on purpose.11 min
TransactionsAll of it or none of it, and what "isolation" is protecting you from.9 min
IndexesWhy a query gets slow, what an index actually is, and how to stop one being used by accident.8 min