Filter by count from another table - sql

Question

This query works fine only without WHERE, otherwise there is an error:
column "cnt" does not exist
SELECT
*,
(SELECT count(*)
FROM B
WHERE A.id = B.id) AS cnt
FROM A
WHERE cnt > 0

Answer 1

Use a subquery:
SELECT a.*
FROM (SELECT A.*,
(SELECT count(*)
FROM B
WHERE A.id = B.id
) AS cnt
FROM A
) a
WHERE cnt > 0;
Column aliases defined in the SELECT cannot be used by the WHERE (or other clauses) for that SELECT.
Or, if the id on a is unique, you can more simply do:
SELECT a.*, COUNT(B.id)
FROM A LEFT JOIN
B
ON A.id = B.id
GROUP BY A.id
HAVING COUNT(B.id) > 0;
Or, if you don't really need the count, then:
select a.*
from a
where exists (select 1 from b where b.id = a.id);

Answer 2

Assumptions:
You need all columns from A in the result, plus the count from B. That's what your demonstrated query does.
You only want rows with cnt > 0. That's what started your question after all.
Most or all B.id exist in A. That's the typical case and certainly true if a FK constraint on B.id references to A.id.
Solution
Faster, shorter, correct:
SELECT * -- !
FROM (SELECT id, count(*) AS cnt FROM B) B
JOIN A USING (id) -- !
-- WHERE cnt > 0 -- this predicate is implicit now!
Major points
Aggregate before the join, that's typically (substantially) faster when processing the whole table or major parts of it. It also defends against problems if you join to more than one n-table. See:
Aggregate functions on multiple joined tables
You don't need to add the predicate WHERE cnt > 0 any more, that's implicit with the [INNER] JOIN.
You can simply write SELECT *, since the join only adds the column cnt to A.* when done with the USING clause - only one instance of the joining column(s) (id in the example) is added to the out columns. See:
How to drop one join key when joining two tables
Your added question in the comment
postgres really allows to have outside aggregate function attributes that are not behind group by?
That's only true if the PK column(s) is listed in the GROUP BY clause - which covers the whole row. Not the case for a UNIQUE or EXCLUSION constraint. See:
Return a grouped list with occurrences using Rails and PostgreSQL
SQL Fiddle demo (extended version of Gordon's demo).