Eliminating duplicate values based on only one column of the table

问题

My query:

SELECT sites.siteName, sites.siteIP, history.date
FROM sites INNER JOIN
     history ON sites.siteName = history.siteName
ORDER BY siteName,date

First part of the output:

How can I remove the duplicates in siteName column? I want to leave only the updated one based on date column.

In the example output above, I need the rows 1, 3, 6, 10

回答1:

This is where the window function row_number() comes in handy:

SELECT s.siteName, s.siteIP, h.date
FROM sites s INNER JOIN
     (select h.*, row_number() over (partition by siteName order by date desc) as seqnum
      from history h
     ) h
    ON s.siteName = h.siteName and seqnum = 1
ORDER BY s.siteName, h.date

回答2:

From your example it seems reasonable to assume that the siteIP column is determined by the siteName column (that is, each site has only one siteIP). If this is indeed the case, then there is a simple solution using group by:

select
  sites.siteName,
  sites.siteIP,
  max(history.date)
from sites
inner join history on
  sites.siteName=history.siteName
group by
  sites.siteName,
  sites.siteIP
order by
  sites.siteName;

However, if my assumption is not correct (that is, it is possible for a site to have multiple siteIP), then it is not clear from you question which siteIP you want the query to return in the second column. If just any siteIP, then the following query will do:

select
  sites.siteName,
  min(sites.siteIP),
  max(history.date)
from sites
inner join history on
  sites.siteName=history.siteName
group by
  sites.siteName
order by
  sites.siteName;

来源：https://stackoverflow.com/questions/17507887/eliminating-duplicate-values-based-on-only-one-column-of-the-table