SQL Query Execution Order

You write SELECT first. PostgreSQL runs it fifth. Here is the actual logical order and why it matters.

Written Order vs. Execution Order

SQL reads top to bottom like English, but that is not the order it logically runs. PostgreSQL processes clauses in a fixed sequence regardless of the order in which they appear on the page.

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

EXAMPLE QUERY

SELECT
  channel,
  AVG(spend) AS avg_spend
FROM marketing.campaigns
WHERE status = 'active'
GROUP BY channel
HAVING AVG(spend) > 48000
ORDER BY avg_spend DESC
LIMIT 2;

Watch the query run against Queryflo campaign data. The numbered step shows the clause being evaluated, not the line currently being read.

Watch the execution order runStage 1 of 7

1FROM

2WHERE

3GROUP BY

4HAVING

5SELECT

6ORDER BY

7LIMIT

FROM marketing.campaigns

name	channel	spend	status
Spring Launch	google_ads	55,000.00	active
Retention Webinar	email	45,000.00	active
Finance Retargeting	linkedin	50,000.00	active
Enterprise Search	google_ads	60,000.00	active

FROM loads all four rows from marketing.campaigns before any condition is checked.

Why This Matters

Two consequences follow directly from the logical order:

You cannot reference a SELECT alias inside WHERE, because WHERE runs before SELECT creates that alias.
You can reference the alias inside ORDER BY, because SELECT has already run by then.

Now You Try

Practice this concept

Marketing wants the two active campaign channels whose average spend is greater than 48,000, ordered from highest to lowest average spend.

Available schema

marketing

Prefix tables with marketing.table_name.

channelavg_spend

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

query.sql

Beginner business practice