SQL JOIN Using WHERE
An older style: list both tables in FROM separated by a comma, and state the join condition in WHERE instead of ON — worth recognizing, not writing.
What & Why
Before JOIN ... ON became standard, the same result was achieved by listing multiple tables in FROM, comma-separated, with the matching condition in WHERE instead — sometimes called an "implicit join." It still works today, but is outdated style.
See How It Works
Marketing reads an older query that links campaign names to lead ids through a WHERE condition.
| 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 |
| 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 |
SELECT
c.name AS campaign_name,
l.id AS lead_id
FROM marketing.campaigns c, marketing.leads l
WHERE l.campaign_id = c.id
ORDER BY lead_id;| campaign_name | lead_id |
|---|---|
| Spring Launch | 301 |
| Spring Launch | 302 |
| Retention Webinar | 303 |
| Finance Retargeting | 304 |
The legacy WHERE equality produces the same four key matches as an inner join.
Practice this concept
Marketing is reviewing a legacy comma join. Return campaign_name and lead_id for every campaign-lead key match, ordered by lead id.
marketingPrefix tables with marketing.table_name.
campaign_namelead_idmarketing.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 |
| 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 |
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