WHERE clause does not find column after a CTE?

Question

New to CTE's and subqueries in SQL.
I have 3 tables:

categories (category_code, category)
countries (country_code, country, continent)
businesses (business, year_founded, category_code, country_code)

Goal is to look at oldest businesses in the world. I used a CTE:

WITH bus_cat_cont AS (
    SELECT business, year_founded, category, country,   
           continent
    FROM businesses AS b
    INNER JOIN categories AS c1
    ON b.category_code = c1.category_code
    INNER JOIN countries AS c2
    ON b.country_code = c2.country_code
    )
SELECT continent,
       category,
       COUNT(business) AS n
FROM bus_cat_cont
WHERE n > 5
GROUP BY continent, category
ORDER BY n DESC;

The code works without WHERE n > 5. But after adding that, I get the error:

column "n" does not exist

I realized there is a much easier way to get the output I want without a CTE.
But I'm wondering: Why do I get this error?

Answer 1

This would work:

WITH bus_cat_cont AS (
   SELECT business, year_founded, category, country, continent
   FROM   businesses AS b
   JOIN   categories AS c1 ON b.category_code = c1.category_code
   JOIN   countries  AS c2 ON b.country_code = c2.country_code
   )
SELECT continent, category, count(business) AS n
FROM   bus_cat_cont
-- WHERE  n > 5                -- wrong
GROUP  BY continent, category
HAVING count(business) > 5     -- right
ORDER  BY n DESC;

The output column name "n" is not visible (yet) in the WHERE or HAVING clause. Consider the sequence of events in an SQL query:

• Best way to get result count before LIMIT was applied

For the record, the result has no obvious connection to your declared goal to "look at oldest businesses in the world". year_founded is unused in the query.
You get the most common continent/category combinations among businesses.

Aside, probably better:

SELECT co.continent, ca.category, n
FROM  (
   SELECT category_code, country_code, count(*) AS n
   FROM   businesses
   GROUP  BY 1, 2
   HAVING count(*) > 5
   ) b
JOIN   categories ca USING (category_code)
JOIN   countries  co USING (country_code)
ORDER  BY n DESC;

There is really no need for a CTE.

Aggregate first, join later. See:

• Query with LEFT JOIN not returning rows for count of 0

Beside being faster, this is also safer. While category_code, country_code should be defined UNIQUE, the same may not be true for continent and category. (You may want to output codes additionally to disambiguate.)

count(*) is implemented separately and slightly faster - and equivalent while business is defined NOT NULL.