Right join isn't pulling in all rows from right table - sql-server

Question

I've got two tables for a call center in a Microsoft SQL Server database, fact_queue, that has the number of calls received, and dim_interval, which is used to convert the interval number (0-95) into time stamps (e.g. 07:15-07:30). It's set up this way so you can easily change the timezone data is being pulled in.
I'm trying to get a result which will show me all 96 intervals regardless of if there's a call or not, but it's not working as expected.
Here's an example of what's in the tables:
Fact_Queue
date_id
queue_id
interval_id
calls_offered
7780
40
0
1
7780
40
2
5
7780
40
3
6
7780
40
5
10
Dim_Interval
interval_id
interval_name
0
00:00 - 00:15
1
00:15 - 00:30
2
00:30 - 00:45
3
00:45 - 01:00
--
--
95
23:45 - 24:00
I've played around with a couple variations of a query and I believe the following should work, but it isn't
SELECT dim_interval.interval_name
,fact_queue.offered_calls
FROM dim_interval
RIGHT JOIN fact_queue
ON fact_queue.interval_id = dim_interval.interval_id
WHERE fact_queue.date_id= '7780'
AND fact_queue.queue_id = '40'
ORDER BY dim_interval.interval_id
This just results in
interval_name
calls_offered
00:00 - 00:15
1
00:30 - 00:45
5
00:45 - 01:00
6
01:15 - 01:30
10
but what I want is
interval_name
calls_offered
00:00 - 00:15
1
00:15 - 00:30
null
00:30 - 00:45
5
00:45 - 01:00
6
01:00 - 01:15
null
Why is the query not working? If it matters I'm using DBeaver version 21.0.3.202104181339

Answer 1

In the snippet dim_interval JOIN fact_queue, you've place the dimension table on the left, not the right. As you want all the dimension table's rows, this means you want a left outer join...
FROM dim_interval LEFT JOIN fact_queue
That only gets you half way there though, because the WHERE clause is applied After the join. This means the WHERE clause would filter out the results which have NULLs.
So, you need to do the filtering During the join...
SELECT dim_interval.interval_name
,fact_queue.offered_calls
FROM dim_interval
LEFT JOIN fact_queue
ON fact_queue.interval_id = dim_interval.interval_id
AND fact_queue.date_id= '7780'
AND fact_queue.queue_id = '40'
ORDER BY dim_interval.interval_id
Some people prefer to do the filtering Before the join, but that's not necessary and actually yields the same execution plan...
SELECT dim_interval.interval_name
,fact_queue.offered_calls
FROM dim_interval
LEFT JOIN (
SELECT *
FROM fact_queue
WHERE date_id= '7780'
AND queue_id = '40'
) AS fact_queue
ON fact_queue.interval_id = dim_interval.interval_id
ORDER BY dim_interval.interval_id