SQL Primary Keys and Foreign Keys
The two concepts that make every join in this series possible — a unique identifier on one side, and a reference to it on the other.
What & Why
A primary key (PK) uniquely identifies each row in a table — no two rows can share the same value, and it can't be NULL. A foreign key (FK) is a column in one table holding a primary key value from another table, creating the actual link a join relies on.
See How It Works
BUSINESS QUESTION
Marketing uses campaigns.id as the parent key and leads.campaign_id as its declared foreign key.
| id | campaign_id | 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 | 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 | US |
| 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 | 45000.00 | 2024-02-10 | 2024-04-15 | active | enterprise | |
| 3 | Finance Retargeting | 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 |
EXAMPLE QUERY
SELECT
l.id AS lead_id,
l.campaign_id,
c.name AS campaign_name
FROM marketing.leads l
JOIN marketing.campaigns c
ON c.id = l.campaign_id
ORDER BY lead_id;RESULT — exact output from the displayed Queryflo rows
| lead_id | campaign_id | campaign_name |
|---|---|---|
| 301 | 1 | Spring Launch |
| 302 | 1 | Spring Launch |
| 303 | 2 | Retention Webinar |
| 304 | 3 | Finance Retargeting |
Every displayed foreign key resolves to its campaign primary key.
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