inner query without FROM - postgresql

Question

I made a mistake when writing the query, I wrote a subquery without a FROM clause:
select * from foo where id in (select id where type 'mm')
I lost half a day to find a mistake because it worked!
I checked the documentation but I did not find any information why ...
It is more interesting:
select * from abc a
inner join abc b on b.id = a.id
where a.id in (select id from (select a.id, row_number () over () lp where b.name = 'abc') x where lp = 1);
http://sqlfiddle.com/#!15/4bb29/12
Can someone explain how it works?

Answer 1

Your first query will certainly not work as it is.
To understand your second query, keep in mind that
A SELECT without a FROM clause is like as if there is a FROM clause with a table that has no column, but one row.
You can reference columns from the outer query in the subquery.
Here is an attempt to explain your query:
SELECT * FROM abc a
/* ok, now we have a table with alias "a" */
INNER JOIN abc b ON b.id = a.id
/* now we also have a table with alias "b" */
WHERE a.id IN
(SELECT id
FROM (SELECT a.id,
/* this will count the one "artificial" row */
row_number() OVER () lp
/* Here you reference the table with alias "b" above.
This is constant as far as the subquery is concerned,
so if it is not TRUE, the subquery will return an
empty result */
WHERE b.name = 'abc') x
/* this is always true, since there is only one line in
the above subquery */
WHERE lp = 1);