DISTINCT ON column only for non null values
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06-03-2021 - |
سؤال
How should I declare a query to only use DISTINCT on not null values for certain column but still keep the records for which the column value is null, I'm trying to modify the following query:
I'm trying to modify the following query,
So, basically I want the second query to return all the messages grouped by parent_id
when the parent_id
column IS NOT NULL and return ALL the records when parent_id
IS NULL.
I'm using PG 9.0.4 and Rails 3.1 - any help would be appreciated, thanks!
المحلول
Select Distinct ON (parent_id) *
from messages
WHERE parent_id IS NOT NULL
UNION
Select * from messages where parent_id IS NULL
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