SQL Foundations

What & Why

SQL is the language used to query relational databases. Every query starts with what you want (SELECT), where to get it (FROM), and — optionally — how to filter (WHERE), sort (ORDER BY), and cap the output (LIMIT). These four clauses cover 80% of day-to-day analytical work.

See how it works

Find the 5 most recently activated Pro users on Queryflo.

SELECT
  id,
  country,
  channel,
  activated_at
FROM growth.users
WHERE plan = 'pro'
  AND activated_at IS NOT NULL
ORDER BY activated_at DESC
LIMIT 5;

Returns 5 rows — one per most-recently-activated Pro user, with their country, acquisition channel, and activation timestamp.

The four clauses

See explanation

SELECT — choose your columns. SELECT tells the database which columns to return.

-- Every column (use sparingly on large tables)
SELECT *
FROM growth.users;

-- Only what you need
SELECT id, country, plan
FROM growth.users;

-- Rename a column with AS
SELECT id, created_at AS joined_at
FROM growth.users;

* means "all columns" — fine for exploration, avoid in production code.
AS gives a column an alias in the output — no effect on the underlying data.

WHERE — filter rows. WHERE keeps only the rows that match a condition.

SELECT id, country, plan
FROM growth.users
WHERE plan = 'pro';

Common operators:

Operator	Example	Meaning
`=`	`plan = 'pro'`	exact match
`!=` or `<>`	`plan != 'free'`	not equal
`>` / `<`	`created_at > '2025-01-01'`	greater / less than
`BETWEEN`	`created_at BETWEEN '2025-01-01' AND '2025-06-30'`	inclusive range
`IN`	`plan IN ('pro','enterprise')`	any of a list
`LIKE`	`country LIKE 'United%'`	pattern match (`%` = wildcard)
`IS NULL`	`churned_at IS NULL`	missing value

Combine with AND / OR:

SELECT id, country
FROM growth.users
WHERE plan = 'pro'
  AND churned_at IS NULL;

ORDER BY — sort the results.

SELECT id, country, activated_at
FROM growth.users
WHERE plan = 'pro'
ORDER BY activated_at DESC;   -- most recently activated first

ASC (default) = smallest → largest.
DESC = largest → smallest.
Sort on multiple columns: ORDER BY country ASC, activated_at DESC.

LIMIT — cap the output.

SELECT id, country, activated_at
FROM growth.users
ORDER BY activated_at DESC
LIMIT 10;   -- 10 most recently activated users

Always add LIMIT when exploring — it keeps queries fast and your results readable.

Now You Try

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