Select count while count is greater than a specific number in postgres sql

Question

I want to get last month records from table. I have tried:

SELECT count(*) as numberOfRows from Table where created_at >  CURRENT_DATE - INTERVAL '1 months'

It's Ok, but I want to add some conditions:

If numberOfRows >= 10, do nothing (numberOfRows can be 20, 30, ...)

else if numberOfRows < 10, select from this table until numberOfRows = 10 (last 2 months, 3 months, etc...).

How can I do that? Thanks in advances!

Answer 1

WITH curr_month_cnt AS (
    SELECT COUNT(*) AS cnt
    FROM your_table
    WHERE created_at > CURRENT_DATE - INTERVAL '1 months'
)

SELECT *
FROM your_table
WHERE created_at > CURRENT_DATE - INTERVAL '1 months'
UNION ALL
SELECT t.*
FROM
(
    SELECT *
    FROM your_table
    WHERE
        created_at <= CURRENT_DATE - INTERVAL '1 months' AND
        (SELECT cnt FROM curr_month_cnt) < 10
    ORDER BY created_at desc
    LIMIT
        GREATEST(0, 10 - (SELECT cnt FROM curr_month_cnt))
) t

This will return a maximum of 10 records, starting with the most recent month and going backwards. In the event that the latest month have not 10 records, then two and three months old data would be returned, in that order.

Answer 2

Based on your description, you would seem to want:

select greatest(10, count(*)) as numberOfRows
from Table
where created_at >  CURRENT_DATE - INTERVAL '1 months';

This seems rather surprising. Perhaps you want:

select (case when sum( (CURRENT_DATE - INTERVAL '1 months' ) :: int) >= 10 
             then sum( (CURRENT_DATE - INTERVAL '1 months' ) :: int)
             else least(10, count(*))
        end) as numberOfRows
from Table
where created_at > CURRENT_DATE - INTERVAL '1 months';