Using DISTINCT to get a unique SQL column in an INNER JOIN

Question

Hello I am trying to run the following query to select one of each host_id, but filter out the other rows if the host_id is the same. However, even when using distinct, I get all of the rows. Can someone help me understand why that is? How would I be able to change this so that only the host_id is unique? Thank you.

SELECT DISTINCT host_id hostid, host.description customer_name, data_template_data.data_source_path file, host.hostname host_name
FROM data_local
JOIN data_template_data ON data_local.id = data_template_data.local_data_id
LEFT JOIN host ON data_local.host_id = host.id
WHERE data_template_data.data_template_id = 41
AND host.disabled=''
AND data_template_data.data_source_path IS NOT NULL

Answer 1

You need aggregation or window functions. Possibly the simplest method is:

with h as (
      <your query here>
     )
select h.*
from (select h.*,
             row_number() over (partition by host_id order by host(id) as seqnum
      from h
     ) h
where seqnum = 1;