Lesson 03 / 9·8 minFree

WHERE: Filtering Your Results

Only show rows that match a condition — the most useful clause in SQL

SELECT * gives you everything. Most of the time you want something more specific: just the paid users, just orders from this week, just products under £20. WHERE is how you filter.

The mental model

WHERE in action

-- Only pro users
SELECT name, email FROM users WHERE plan = 'pro';

-- Orders over £50
SELECT * FROM orders WHERE amount > 50;

-- Users who signed up after a certain date
SELECT name FROM users WHERE created_at > '2026-01-01';

-- Find a specific user by id
SELECT * FROM users WHERE id = 42;

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WHERE is a bouncer with a clipboard

Imagine every row in your table walking up to a bouncer. The bouncer has one rule: "plan must equal pro." Sarah arrives — plan=pro. In she goes. Alex arrives — plan=free. Turned away. Priya arrives — plan=pro. In she goes. The WHERE clause is that bouncer. Only rows that pass the condition get into the result.

Comparison operators

= (equals) — WHERE plan = 'pro' — exact match. Note: SQL uses single quotes for text, not double.
!= or <> (not equal) — WHERE plan != 'free' — everything except free.
> < >= <= (numeric) — WHERE amount >= 100 — orders of £100 or more.
LIKE (pattern matching) — WHERE email LIKE '%@gmail.com' — all Gmail users. % is a wildcard matching anything.
IN (list of values) — WHERE plan IN ('pro', 'enterprise') — either plan type.
IS NULL / IS NOT NULL — WHERE cancelled_at IS NULL — only rows where that column has no value.
BETWEEN — WHERE amount BETWEEN 10 AND 50 — inclusive range.

Combining conditions: AND, OR, NOT

Multiple conditions in WHERE

-- Pro users who signed up this year
SELECT name FROM users
WHERE plan = 'pro'
  AND created_at >= '2026-01-01';

-- Free OR trial users
SELECT name FROM users
WHERE plan = 'free'
   OR plan = 'trial';

-- All users except admins
SELECT name FROM users
WHERE NOT role = 'admin';

-- Complex: active pro users with orders over £100
SELECT u.name FROM users u
JOIN orders o ON o.user_id = u.id
WHERE u.plan = 'pro'
  AND o.amount > 100
  AND o.status = 'paid';

WHERE runs before SELECT

SQL processes FROM first (get the table), then WHERE (filter rows), then SELECT (pick columns). This matters when debugging: if your WHERE clause is wrong, your SELECT result will be wrong too. Always check you're filtering what you think you're filtering.

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Try this

Write three WHERE queries: (1) all pro users, (2) all orders over £30, (3) all users whose email contains "gmail". Then combine them: all pro users who placed an order over £30 — this will require a JOIN (coming in lesson 6).

SELECT and FROM: Asking Questions ORDER BY and LIMIT: Sorting and Paging