问题
I'm pretty sure this has been asked before but I am struggling to get the correct syntax for a table containing data like
id date type report item_id
1 2018-11-07 Veröffentlichung des 9-Monats-Berichtes TRUE 16
2 2018-11-06 Veröffentlichung des 9-Monats-Berichtes TRUE 17
3 2019-03-07 Veröffentlichung des Jahresberichtes TRUE 17
4 2019-05-10 Bericht zum 1. Quartal TRUE 17
The query I am trying to formulate is
SELECT date, AGE(now(), date) as t1
FROM dates
WHERE t1 > 0
Meaning I am only looking for values in the past.
However, I get an error
ERROR: column "t1" does not exist
(of course, it is an alias). Does Postgresql not support aliases here?
回答1:
You cannot refer to alias in WHERE condition, because logically WHERE is executed before SELECT.
You could use subquery:
SELECT *
FROM (SELECT date, AGE(now(), date) as t1
FROM dates) sub
WHERE sub.t1 > interval '0::seconds';
Or LATERAL(my favourite way):
SELECT date, s.t1
FROM dates
,LATERAL (SELECT AGE(now(), date) as t1) AS s
WHERE s.t1 > interval '0::seconds';
Or repeat expression(violates DRY principle):
SELECT date, AGE(now(), date) as t1
FROM dates
WHERE AGE(now(), date) > interval '0::seconds';
As for calculating AGE you don't really need it, because you could rewrite it as date > now().
Related articles:
PostgreSQL: using a calculated column in the same query
MySQL - Search into a custom Column
Why do “linq to sql” queries starts with the FROM keyword unlike regular SQL queries?
回答2:
If you're in a hurry and have an (ordered) index on date don't do that.
Because this query can use the index giving a massive gain in performance at only a slight investment in coding effort.
SELECT date, AGE(now(), date) AS t1
FROM dates
WHERE date > now();
I say now(), because you did, but perhaps you want CURRENT_DATE instead
To create a suitable index do
create index dates_date on dates(date);
来源：https://stackoverflow.com/questions/52576569/postgresql-where-with-age-function