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Given the following select that will count related entities for every record

SELECT MAIN.*, SUB.app_count
FROM my_table_view MAIN
     LEFT JOIN (SELECT MAIN2.p_key,
                       count(DISTINCT T3.app_id) AS app_count
                FROM my_table MAIN2
                         JOIN T1 ...
                         LEFT JOIN T2 ...
                         LEFT JOIN T3 ...
                GROUP BY MAIN2.p_key) SUB ON MAIN.p_key = SUB.p_key
WHERE ...

Trying to simplify it, came up with the following :

SELECT MAIN.p_key, count(DISTINCT T3.app_id) AS app_count -- select MAIN.* errors out - missing GROUP BY
FROM my_table_view MAIN
     LEFT JOIN T1 ON ...
     LEFT JOIN T2 ON ...
     LEFT JOIN T3 ON ...
WHERE ...
GROUP BY MAIN.p_key

How to select all data from the MAIN table without adding a bunch of GROUP BYs ?
Would also appreciate performance hints.

Edit

I think this is related
Postgres GROUP BY id works on table directly, but not on identical view

2
  • Remove GROUP BY at all. Use window version of COUNT(). Add DISTINCT for to remove duplicates from output rowset. PS. This is NOT a primary key - you must look at query data source, not at a particular table in it.
    – Akina
    Jun 30 '20 at 5:20
  • @Akina do you mind writing a response ?
    – Bax
    Jul 1 '20 at 17:30
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If p_key is the primary key of my_table, it should not be necessary to add the other columns, unless you want the statement to comply with the SQL standard.

2
  • p_key is the primary key but I select from a view
    – Bax
    Jun 30 '20 at 23:13
  • Then add all the columns to the GROUP BY clause. It is the safe and standard way. Jul 1 '20 at 6:07

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