Can you tell if a subquery be inserted by any other method except using aggregate function or relational operator

Question

Sql subquery use

Answer 1

You can use subqueries to construct new tables you select from
SELECT * FROM
(
SELECT a.X, a.Y FROM a_table AS a
WHERE a.Z > 3
) as _;
(no aggregation, no relational operator)

Answer 2

common table expression (CTE):
WITH TEMP
AS
(
SELECT X.FIELD_A
,X.FIELD_B
FROM TABLE X
)
SELECT TEMP.FIELD_A
, TEMP.FIELD_B
FROM TEMP
;

Answer 3

yes