How can I combine multiple rows into one column using PostgreSQL?

问题

How can I combine multiple results into one column?

For example, my query displays this result:

internaldocid | document id | versionid | title | tagname
1146 | DOC-1146 | 2 | Press Release | tag1
1146 | DOC-1146 | 2 | Press Release | tag2
1146 | DOC-1146 | 2 | Press Release | tag3

I would like it to display this result:

internaldocid | document id | versionid | title | tagname
1146 | DOC-1146 | 2 | Press Release | tag1, tag2, tag3

Here is my SQL:

SELECT
  cmsdocument.internaldocid,
  cmsdocument.documentid,
  cmsdocument.versionid,
  cmsdocversion.title,
  cmstag.tagname
FROM 
  public.cmsobjecttag,
  public.cmstag,
  public.cmsdocument,
  public.cmsdocversion,
  public.cmsuser
WHERE 
  cmsobjecttag.tagid = cmstag.tagid AND
  cmsobjecttag.objectid = cmsdocument.internaldocid AND
  cmsdocument.internaldocid = cmsdocversion.internaldocid AND
  cmsdocument.userid = cmsuser.userid AND
  cmsdocversion.state = 'published' AND
  cmsobjecttag.objecttype = '102';

Any help would be greatly appreciated.

回答1:

Since PostgreSQL version 9, this is done simply by using string_agg:

SELECT
  cmsdocument.internaldocid,
  cmsdocument.documentid,
  cmsdocument.versionid,
  cmsdocversion.title,
  string_agg(cmstag.tagname, ',')
FROM 
  public.cmsobjecttag,
  public.cmstag,
  public.cmsdocument,
  public.cmsdocversion,
  public.cmsuser
WHERE 
  cmsobjecttag.tagid = cmstag.tagid AND
  cmsobjecttag.objectid = cmsdocument.internaldocid AND
  cmsdocument.internaldocid = cmsdocversion.internaldocid AND
  cmsdocument.userid = cmsuser.userid AND
  cmsdocversion.state = 'published' AND
  cmsobjecttag.objecttype = '102'
GROUP BY 
  cmsdocument.internaldocid,
  cmsdocument.documentid,
  cmsdocument.versionid,
  cmsdocversion.title

来源：https://stackoverflow.com/questions/29832472/how-can-i-combine-multiple-rows-into-one-column-using-postgresql

标签

sql

postgresql

aggregate-functions