postgreSQL and joining on alias - postgresql

Question

I have a query that, in very simplified form, looks like this:
SELECT aa, bb, xx
FROM (VALUES ('a1', 'b1'), ('a2', 'b2')) as T1(aa, bb)
LEFT JOIN (
SELECT xx FROM (VALUES
('a1', 'y1'), ('x2', 'y2')) as T2(xx, yy)
) as T3 ON T1.aa = T3.xx ;
which executes to produce this:
aa | bb | xx
----+----+--------
a1 | b1 | a1
a2 | b2 | (NULL)
(2 rows)
In real life the column xx is defined using a case statement and alias:
SELECT
...,
(CASE WHEN rgr.age_years < 18 THEN '< 18'
WHEN rgr.age_years < 36 THEN '26-35'
WHEN rgr.age_years < 46 THEN '36-45'
WHEN rgr.age_years < 56 THEN '46-55'
WHEN rgr.age_years < 130 THEN '> 55' END)
as row_name,
...
FROM
(VALUES ('< 18'), ('26-35'), ('36-45'),
('46-55'), ('> 55'))
as prn(possible_row_name)
LEFT JOIN other_table rgr
ON prn.possible_row_name = ??row_name??
Obviously ??row_name?? is an alias and so is not available at the time I specify the join. But I've not found the right formulation.
The point is simply that I'm doing a select on table rgr and I want all the age tranches present, even if no entity satisfies that number.

Answer 1

You can create the column on the fly inside the table expression rgr, as in:
SELECT
...,
rgr.row_name,
...
FROM
(VALUES ('< 18'), ('26-35'), ('36-45'),
('46-55'), ('> 55'))
as prn(possible_row_name)
LEFT JOIN ( -- table expression rgr defined here
select t.*,
CASE WHEN rgr.age_years < 18 THEN '< 18'
WHEN rgr.age_years < 36 THEN '26-35'
WHEN rgr.age_years < 46 THEN '36-45'
WHEN rgr.age_years < 56 THEN '46-55'
WHEN rgr.age_years < 130 THEN '> 55' END
as row_name
from other_table t
) rgr ON prn.possible_row_name = rgr.row_name