如何左加入一个表而不保留左表中的所有记录？

Question

I have three tables:

1 - Dinners, with dinners details

dinnerid | description | date
   1234
   5678
   9012

2 - dinners_detgroups, with the groups that are participating to the dinners

dinnerid | groupid
1234       4897
1234       5643
5678       2398

3- groups_users, with the users that are part of groups

groupid | userid
4897      48
4897      97
5643      56
5643      43

I'm trying to make a query that returns the details of a dinner with all the groupids that are participating in which there is the userid I'm looking for

For example: userid 48 ---> groupid 4897 ---> dinnerid 1234

I want that the query returns the detail of the dinner with id 1234 and the groupid that are participating (in this case 4897 5643)

I've tried with this

SELECT dinners.*, array_agg(dinners_detgroups.groupid) AS groupid 
FROM dinners 
  INNER JOIN dinners_detgroups ON dinners.dinnerid = dinners_detgroups.dinnerid 
  LEFT JOIN groups_users 
    ON dinners_detgroups.groupid = groups_users.groupid 
    AND groups_users.userid = ' + request.user.userid + ' 
GROUP BY dinners.dinnerid

But returns me all the dinners, not only the dinner in which there is user 48.

and with this

SELECT dinners.*, array_agg(dinners_detgroups.groupid) AS groupid 
FROM dinners 
  INNER JOIN dinners_detgroups ON dinners.dinnerid = dinners_detgroups.dinnerid 
  INNER JOIN groups_users 
          ON dinners_detgroups.groupid = groups_users.groupid 
         AND groups_users.userid = ' + request.user.userid + ' 
GROUP BY dinners.dinnerid

That returns the right dinner but only the groupid in which is user id 48 (groupid 4897) but i want all the groupid that are participating to the dinner (in this case also groupid 5643)

Answer 1

If I understand correctly, you want all groups to be included when one of them has the requested user. If so, I think you can use this method:

SELECT d.*, array_agg(DISTINCT dd.groupid) AS groupid
FROM dinners d INNER JOIN
     dinners_detgroups dd
     ON d.dinnerid = dd.dinnerid JOIN
     groups_users gu
     ON dd.groupid = gu.groupid 
GROUP BY d.dinnerid
HAVING bool_or(gu.userid = ?)