SQL Statement in Webi 3.x and Webi 4.x

Question

Hello experts,
I am facing with a problem regarding an SQL Statement written in Query Panel as a Custom SQL. The bellow statement is executed in Webi 3.x but not in Webi 4.x.. Webi 4.x does not recognize the statement from line 4 (SELECT within a SELECT), but in Webi 3.x everything is OK.
Have you faced with a similar problem ?
Thank's a lot for your answers.
1 SELECT
2   a.doc_id,
3   a.doc_type,
4   (SELECT b.doc_date FROM test b WHERE b.doc_type = 'I' and b.doc_id = a.doc_ref_id)
5 FROM
6   test a
7 WHERE
8   a.doc_type = 'F'

Answer 1

Thank you very much for your answer. Very very interesting these links. I will read because I am very curious to find out the answer.
In the meanwhile, I solved the problem in the following manner:
SELECT
a.doc_id,
a.doc_type,
b.doc_date
FROM
test a,
(SELECT * FROM test WHERE doc_type = 'I') b
WHERE
a.doc_ref_id = b.doc_id and a.doc_type = 'F'
All the best,
    Marius