Question

I have a table, lets say “Records” with structure:

id      date
--      ----
1       2012-08-30
2       2012-08-29
3       2012-07-25

I need to write an SQL query in PostgreSQL to get record_id for MIN date in each month.

month    record_id
-----    ---------
8           2
7           3

as we see 2012-08-29 < 2012-08-30 and it is 8 month, so we should show record_id = 2

I tried something like this,

SELECT
   EXTRACT(MONTH FROM date) as month,
   record_id,
   MIN(date) 
FROM Records
GROUP BY 1,2

but it shows 3 records.

Can anybody help?

Answer 1

This will return multiples if you have duplicate minimum dates:

Select
  minbymonth.Month,
  r.record_id
From (
  Select
    Extract(Month From date) As Month, 
    Min(date) As Date
  From
    records
  Group By 
    Extract(Month From date)
  ) minbymonth
    Inner Join
  records r
    On minbymonth.date = r.date
Order By
  1;

Or if you have CTEs

With MinByMonth As (
  Select
    Extract(Month From date) As Month, 
    Min(date) As Date
  From
    records
  Group By 
    Extract(Month From date)
  )
Select
  m.Month,
  r.record_id
From
  MinByMonth m
    Inner Join
  Records r
    On m.date = r.date
Order By
  1;

http://sqlfiddle.com/#!1/2a054/3