Subject: Re: Top 3 values for each group in PGSQL
To: pgsql-sql@lists.postgresql.org
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From: Achilleas Mantzios <achill@matrix.gatewaynet.com>
Message-ID: <bef5c3a6-7f3c-d8f2-259e-54735dee90f7@matrix.gatewaynet.com>
Date: Fri, 1 Mar 2019 15:17:48 +0200
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On 1/3/19 2:52 μ.μ., Thomas Kellerer wrote:
> Ila B. schrieb am 01.03.2019 um 11:51:
>> Hello,
>>
>> I’m working on a health database and I’m trying to extract the most popular prescription codes from a custom table I structured like this:
>>
>> Year - Code - Count(code)
>>
>> I want to extract the 3 codes with maximum count for each year. I know I should be using rank() but I don’t really understand how this works.
>> I am using pgAdmin4 version 3.5 with PostgreSQL 10.6 on Windows 10 Pro and no permission to update.
> Something along the lines:
>
> select code, year, "count"
> from (
>     select code, year, "count",
>            dense_rank() over (partition by code, year order by "count" desc) as rnk
>     from the_table
> ) t
> where rnk <= 3;
Yup, that's the idea
select qryout.* FROM (select qry.*,dense_rank() OVER (ORDER BY count DESC) FROM (select id_1,year_built,count(*) from vessels WHERE year_built IS NOT NULL AND year_built<>'' group by id_1,year_built 
ORDER BY COUNT(*) DESC) as qry ) qryout WHERE dense_rank<=3;
  id_1 | year_built | count | dense_rank
------+------------+-------+------------
    94 | 2009       |    11 |          1
    97 | 2010       |    10 |          2
    94 | 2011       |    10 |          2
    94 | 1975       |     9 |          3
    94 | 1976       |     9 |          3
(5 rows)

or

select qryout.* FROM (select qry.*,dense_rank() OVER (ORDER BY count DESC) FROM (select distinct id_1,year_built,count(*) OVER (partition by id_1,year_built) from vessels WHERE year_built IS NOT NULL 
AND year_built<>'') as qry ) qryout WHERE dense_rank<=3;
  id_1 | year_built | count | dense_rank
------+------------+-------+------------
    94 | 2009       |    11 |          1
    97 | 2010       |    10 |          2
    94 | 2011       |    10 |          2
    94 | 1975       |     9 |          3
    94 | 1976       |     9 |          3
(5 rows)


>


-- 
Achilleas Mantzios
IT DEV Lead
IT DEPT
Dynacom Tankers Mgmt