How do I get unique user engagements?

问题

I am trying to list unique user engagements with the engagement_type

I used to have the following query.

SELECT u.id, u.fname, u.lname FROM (
  SELECT id, engagement_type FROM (
    SELECT user_id AS id, 'comment'
    FROM comments WHERE commentable_id = 48136 AND commentable_type = 'Video'
    UNION ALL
    SELECT user_id AS id, 'like'
    FROM likes WHERE likeable_id = 48136 AND likeable_type = 'Video'
  ) AS a
  GROUP BY id
  LIMIT 10
) b JOIN users u USING (id);

Returns:

id        | fname      | lname
------------------------------
1         | joe        | abc
2         | sarah      | qer
3         | megan      | tryey
4         | john       | vdfa

Which is fine. Now, I want to include the engagment type. I've come up with this:

SELECT u.id, u.fname, u.lname, engagement_type FROM (
  SELECT id, engagement_type FROM (
    SELECT user_id AS id, 'comment' AS engagement_type
    FROM comments WHERE commentable_id = 48136 AND commentable_type = 'Video'
    UNION ALL
    SELECT user_id AS id, 'like' AS engagement_type FROM likes
    WHERE likeable_id = 48136 AND likeable_type = 'Video'
  ) AS a
  GROUP BY id, engagement_type
  LIMIT 10
) b JOIN users u USING (id);

Which now returns:

id        | fname      | lname    | engagement_type
---------------------------------------------------
1         | joe        | abc      | comment
2         | sarah      | qer      | like
3         | megan      | tryey    | like
4         | john       | vdfa     | like
1         | joe        | abc      | like     
3         | megan      | tryey    | comment

The only problem with above. The results are not unique anymore. As you can see, Joe and Megan have 2 entries.

Any idea how I can get this to work?

回答1:

Simpler with DISTINCT ON in PostgreSQL.
For lack of definition I pick the first engagement_type according to its sort order:

SELECT u.id, u.fname, u.lname, b.engagement_type
FROM  (
   SELECT DISTINCT ON (1)
          id, engagement_type
   FROM (
      SELECT user_id AS id, 'comment' AS engagement_type
      FROM   comments
      WHERE  commentable_id = 48136
      AND    commentable_type = 'Video'

      UNION ALL
      SELECT user_id, 'like'
      FROM   likes
      WHERE  likeable_id = 48136 
      AND    likeable_type = 'Video'
      ) a
   ORDER  BY 1, 2
   LIMIT  10
   ) b
JOIN users u USING (id);

Details, links and explanation:

Select first row in each GROUP BY group?

If you want a unique list of all engagement_types:

  SELECT id, string_agg(DISTINCT engagement_type, ', ') AS engagement_types
   FROM (
      ...
      ) a
   GROUP  BY 1
   ORDER  BY <whatever>
   LIMIT  10;

string_agg() need Postgres 9.0 or later.
This form allows to order by whatever you want, while you'd need another subquery if you want ORDER BY to disagree with DISTINCT ON.

回答2:

If you want only one row you need to remove the engagement_type from the GROUP BY clause. however this then won't show you all the different engagement_type's.

If you want to list the engagement_types in one row without duplicating the user details then use the ARRAY_TO_STRING function. This contaminates the results to one line. So you could list the engagement types from the comments table as a comma separated list.

来源：https://stackoverflow.com/questions/15186763/how-do-i-get-unique-user-engagements

标签

sql

postgresql

subquery