SQL Joins

Combine rows from two or more tables based on a shared column — the foundation of nearly every real-world query, since real data almost never lives in one single table.

What & Why

Real business data is split across related tables. marketing.campaigns stores campaign details while marketing.leads stores lead activity, and a JOIN combines them by matching marketing.leads.campaign_id to marketing.campaigns.id.

The column names do not need to be identical. A foreign key and the primary key it references describe the relationship; similar-looking names alone do not.

See How It Works

BUSINESS QUESTION

Marketing wants campaign names beside lead counts.

id	name	channel	spend	start_date	end_date	status	target_segment
1	Spring Launch	google_ads	55000.00	2024-01-15	2024-03-31	active	smb
2	Retention Webinar	email	45000.00	2024-02-10	2024-04-15	active	enterprise
3	Finance Retargeting	linkedin	50000.00	2024-03-12	2024-05-31	active	enterprise
4	Enterprise Search	google_ads	60000.00	2024-04-01	2024-06-30	active	enterprise

id	campaign_id	email	created_at	qualified_at	converted_at	lead_score	source	country
301	1	ana@example.com	2024-01-21 09:10:00+00	2024-01-22 11:00:00+00	2024-02-02 10:00:00+00	86	google_ads	US
302	1	ben@example.com	2024-01-24 12:40:00+00	NULL	NULL	52	google_ads	CA
303	2	chloe@example.com	2024-02-16 08:30:00+00	2024-02-18 14:20:00+00	NULL	74	email	GB
304	3	dev@example.com	2024-03-20 17:15:00+00	2024-03-21 09:00:00+00	2024-04-04 16:00:00+00	91	linkedin	US

EXAMPLE QUERY

SELECT
  c.name,
  COUNT(l.id) AS leads
FROM marketing.campaigns c
JOIN marketing.leads l ON l.campaign_id = c.id
GROUP BY c.name
ORDER BY leads DESC, c.name ASC;

RESULT — exact output from the displayed Queryflo rows

name	leads
Spring Launch	2
Finance Retargeting	1
Retention Webinar	1

The inner join omits Enterprise Search because it has no lead row.

Intermediate business practice