Unnecessary LEFT JOINs on all Child tables from main LEFT JOIN table required in query for correct result - sql

Question

I am LEFT JOIN-ing the table RECALLS_T with EVENTS_T. Some Recalls do not have any Events, and for those I want a blank row returned.
However, once an entry in EVENTS_T exists, all the following extra tables from EVENTS_T which I also need (ANSWERS_T, ACTIVITY_QUESTIONS_T, ACTIVITES_T) are guaranteed to have entries. For these subsequent tables there is no need to do a LEFT JOIN from their parent, it can be an INNER JOIN just as well.
But if I do the following, the query does not return blank EVENTS_T rows for a RECALLS_T.
SELECT ..
FROM
recalls_t r
LEFT JOIN events_t e ON ( r.id = e.recall_id ) --Only LEFT JOIN this main table
INNER JOIN answers_t ans ON (ans.event_id = e.id)
INNER JOIN activity_questions_t aq ON (ans.activity_question_id = aq.id)
INNER JOIN PUBLIC.activities_t act ON (aq.activity_id = act.id)
Instead, I need to change every subsequent INNER JOIN to a LEFT JOIN as well, in order to get Recalls with blank Events.
SELECT ..
FROM
recalls_t r
LEFT JOIN events_t e ON ( r.id = e.recall_id )
LEFT JOIN answers_t ans ON (ans.event_id = e.id)
LEFT JOIN activity_questions_t aq ON (ans.activity_question_id = aq.id)
LEFT JOIN PUBLIC.activities_t act ON (aq.activity_id = act.id)
I'm just not clear on why I have to do this. The top-level optional join is all I care about, but the downstream joins from EVENTS_T are guaranteed to have data provided the Event exists. Shouldn't the top-level single EVENTS_T join be enough? I'm using Postgres.

Answer 1

The reason you need to do this is because rows preserved by the outer join will have NULL for all columns from events - so inner joining using an equality condition on e.id will remove them from the result again.
you can move the ON clause to the end of the query to give your desired semantics.
The left join is then on the virtual result of inner joining events_t, answers_t, activity_questions_t and activities_t
SELECT ...
FROM recalls_t r
LEFT JOIN events_t e
INNER JOIN answers_t ans
ON ( ans.event_id = e.id )
INNER JOIN activity_questions_t aq
ON ( ans.activity_question_id = aq.id )
INNER JOIN PUBLIC.activities_t act
ON ( aq.activity_id = act.id )
ON ( r.id = e.recall_id )
Or you could consider a RIGHT JOIN here instead
SELECT ...
FROM events_t e
INNER JOIN answers_t ans
ON ( ans.event_id = e.id )
INNER JOIN activity_questions_t aq
ON ( ans.activity_question_id = aq.id )
INNER JOIN PUBLIC.activities_t act
ON ( aq.activity_id = act.id )
RIGHT JOIN recalls_t r
ON ( r.id = e.recall_id )

Answer 2

You have to do this because answers_t`` is joining onevents_s. If thejoinkey (e.recall_id) isNULL`, then there will be no match. And the inner join will not return the row. And so on for the other tables.
You seem to understand the fix. Once you use left join, you need to continue using left join for those tables that are connected to the second table of the left join.