refer to array from SELECT clause in WHERE clause - sql

Question

I have the following two tables:
create table person
(
identifier integer not null,
name text not null,
age integer not null,
primary key(identifier)
);
create table agenda
(
identifier integer not null,
name text not null,
primary key(identifier)
);
They are joined with the following table:
create table person_agenda
(
person_identifier integer not null,
agenda_identifier integer not null,
primary key(person_identifier, agenda_identifier),
foreign key(person_identifier) references person(identifier),
foreign key(agenda_identifier) references agenda(identifier)
);
I am trying to refer to an array, as definied in the SELECT clause, in the WHERE clause.
The following works:
select identifier, name, array(select identifier from agenda a, person_agenda pa where person_identifier = p.identifier and identifier = agenda_identifier and name = '...') as r
from person p;
This does not:
select identifier, name, array(select identifier from agenda a, person_agenda pa where person_identifier = p.identifier and identifier = agenda_identifier and name = '...') as r
from person p
where array_length(r, 1) >= 1;
It says that r is not a known column. How can I refer to this array in the WHERE clause?
The purpose of my second query is to:
omit persons without agendas (by filtering on array_length() >= 1)
get all agenda identifiers, so I can fetch their information in a subsequent query without having to filter again (on field agenda.name in my example above) (by projecting the array in the SELECT clause)
The first bullet can be done with a simple join. But, for the first bullet in combination with the second bullet, I need some kind of aggregation on the agenda identifiers. I thought arrays would be useful for this.
edit
According to user Saba, this is not possible. Thanks for your feedback.
Is the following query a good alternative?
select person.identifier, person.name, array_agg(agenda.identifier)
from person, person_agenda, agenda
where person.identifier = person_identifier and
agenda.identifier = agenda_identifier and
agenda.name = '...'
group by person.identifier;

Answer 1

Alias cant be used in WHERE clause, if you want to use the alias in your WHERE clause, you need to wrap it in a subquery or CTE. Because query will be executed in the following order:
FROM
ON
JOIN
**WHERE**
GROUP BY
WITH CUBE or WITH ROLLUP
HAVING
**SELECT**
DISTINCT
ORDER BY
TOP
Try something like this:
SELECT
FROM (
select identifier, name, array(select identifier from agenda a, person_agenda pa where person_identifier = p.identifier and identifier = agenda_identifier and name = '...') as r
from person p
) AS per
where array_length(r) >= 1;
Your case seems straight forward, the below one should work for you:
select person.identifier, person.name, array_agg(agenda.identifier)
from person, person_agenda, agenda
where person.identifier = person_identifier and
agenda.identifier = agenda_identifier and
agenda.name = '...'
group by person.identifier, person.name;