SQL WHERE: filter rows

A table often holds thousands of rows, when you only want a few of them: the customers of one city, the products under a given price, the cancelled orders… That is exactly what the WHERE clause does: it only keeps the rows that meet a condition.

Syntax

SELECT columns
FROM table
WHERE condition;

How it works

For each row of the table, SQL evaluates the WHERE condition. If it is true, the row is kept; otherwise, it is discarded.

Comparison operators

Text and dates are written between single quotes ('Paris', '2025-01-01'), numbers without.

Combining several conditions

• AND: both conditions must be true.
• OR: at least one of them must be true.
• NOT: reverses a condition.

AND is evaluated before OR, just like multiplication before addition. As soon as you mix the two, add parentheses to be sure of what your query means.

Diagram

Simplified diagram to show the idea. The examples below use the real SQL Quest practice database (a fictional online shop: see the full schema).

Examples with their results

SELECT prenom, nom, ville
FROM clients
WHERE ville = 'Paris';

SELECT nom, prix
FROM produits
WHERE prix < 10;

SELECT prenom, nom, departement, salaire
FROM employes
WHERE departement = 'IT' AND salaire > 4000;

SELECT nom, categorie, prix
FROM produits
WHERE (categorie = 'Sport' OR categorie = 'Maison') AND prix < 15;

Common mistakes

SELECT * FROM clients WHERE ville = Paris;

SELECT * FROM clients WHERE ville = 'Paris';

SELECT * FROM produits WHERE categorie = 'Sport' OR categorie = 'Maison' AND prix < 15;

SELECT * FROM produits WHERE (categorie = 'Sport' OR categorie = 'Maison') AND prix < 15;

SELECT * FROM clients WHERE age = NULL;

SELECT * FROM clients WHERE age IS NULL;

Exercises

Try writing the query yourself before opening the solution.

SELECT * FROM commandes WHERE statut = 'annulée';

SELECT prenom, nom, age FROM clients WHERE ville = 'Lyon' AND age < 30;

Frequently asked questions