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.

Now You Try

Practice this concept

Marketing wants campaign names with their lead counts. Keep only campaigns that have leads, then order by lead count descending and campaign name ascending.

Available schema

marketing

Prefix tables with marketing.table_name.

nameleads

marketing.campaigns

Column	Type
id	integer
name	text
channel	text
spend	numeric
start_date	date
end_date	date
status	text
target_segment	text
legacy_id	text

marketing.leads

Column	Type
id	integer
campaign_id	integer
email	text
created_at	timestamp with time zone
qualified_at	timestamp with time zone
converted_at	timestamp with time zone
lead_score	integer
source	text
country	text
archive_status	text

query.sql

Intermediate business practice