Postgres : using computed variable in a SELECT statement (part 2/2) - sql

Question

I'm having a SELECT statement as follow (doesn't work):
SELECT foo,
extract(day from CAST (date as TIMESTAMP) - CAST (birth_date as TIMESTAMP)) / 365.25 as age_norm,
CASE
WHEN age_norm >= 0 AND age_norm <1 THEN '00'
WHEN age_norm >= 1 AND age_norm <5 THEN '01-4'
--etc
END as age_group
FROM bar
Is there a way to "inject" here the "variable" age_normin the query ?
NOTE
Asked a similar question here, but without the parameter foopresent in this question
EDIT
Tried:
SELECT foo,
(
SELECT t.age_norm,
CASE
WHEN t.age_norm >= 0 AND t.age_norm <1 THEN '00'
WHEN t.age_norm >= 1 AND t.age_norm <5 THEN '01-4'
--etc
END
FROM (SELECT extract(day from CAST (date as TIMESTAMP) - CAST (birth_date as TIMESTAMP)) / 365.25 as age_norm FROM bar) t
)
as age_group
FROM bar
But getting:
ERROR: subquery must return only one column
LINE 10: ( SELECT t.age_norm,
^
SQL state: 42601
Character: 212
CONCLUSION
Tried some of proposed solutions but either the resulting query is too slow (compared to hard coding the extract function in each where clause) or the resulting query become too complicate (it is in fact more complicate than expressed in this post with group by clauses and others things).
So will implements each when clause as follow:
WHEN (extract(day from CAST (date as TIMESTAMP) - CAST (birth_date as TIMESTAMP)) / 365.25) >=0 AND (extract(day from CAST (date as TIMESTAMP) - CAST (birth_date as TIMESTAMP)) / 365.25) <1 THEN '00'
etc.
Thank you for your replies!

Answer 1

Use a derived table (a query in the FROM clause):
SELECT
foo,
age_norm,
CASE
WHEN age_norm >= 0 AND age_norm <1 THEN '00'
WHEN age_norm >= 1 AND age_norm <5 THEN '01-4'
END as age_group
FROM (
SELECT
foo,
extract(day from CAST (date as TIMESTAMP) - CAST (birth_date as TIMESTAMP)) / 365.25 as age_norm
FROM bar
) s

Answer 2

perhaps a lateral join like this....
SELECT foo,age_group.*
FROM
FROM bar
, LATERAL (
SELECT t.age_norm,
CASE
WHEN t.age_norm >= 0 AND t.age_norm <1 THEN '00'
WHEN t.age_norm >= 1 AND t.age_norm <5 THEN '01-4'
--etc
END
FROM (SELECT extract(day from CAST (bar.date as TIMESTAMP) - CAST bar.birth_date as TIMESTAMP)) / 365.25 as age_norm ) t
) as age_group;