Трябва само да направите псевдоним, когато препращате към колона, която не е уникално дефинирана. Това означава, че колоната съществува в повече от една таблица/изведена таблица. Препратка може да бъде в оператора select или съединение. Ако всички колони са уникални, тогава нямате нужда от псевдоним.
Предпочитам да използвам псевдоними през цялото време за яснота и защото помага с Intellisense в PL/SQL.
--ALIAS needed, because the 'a' column referenced is not unique
--this will throw an error
select a, a, b, c
from (select 'A1' as a, 'B1' as b, 'C1' as c from dual),
(select 'A2' as a from dual);
--this will not throw an error
select t1.a, t2.a, b,c
from (select 'A1' as a, 'B1' as b, 'C1' as c from dual) t1,
(select 'A2' as a from dual) t2;
;
--ALIAS not needed for join, because all referenced columns are unique
select a, b, c, d, e, f
from (select 'A' as a, 'B' as b, 'C' as c from dual)
join (select 'D' as d, 'E' as e, 'F' as f from dual)
on a = d;
--ALIAS needed for join, because the 'x' column referenced is not unique
--this will throw an error
select a
from (select 'A' as a, 'B' as b, 'C' as c, 'X' as x from dual)
join (select 'D' as d, 'E' as e, 'F' as f, 'X' as x from dual)
on x = x;
--this will not throw an error
select a
from (select 'A' as a, 'B' as b, 'C' as c, 'X' as x from dual) t1
join (select 'D' as d, 'E' as e, 'F' as f, 'X' as x from dual) t2
on t1.x = t2.x;