Need help understanding a complex query with multiple join conditions - sql

Question

I have a query that I am trying to understand. Can someone shed light on to the details of what this query does?
I've only ever used one ON clause in a join condition. This one has multiple conditions for the LEFT JOIN, making it tricky to understand.
INSERT INTO nop_tbl
(q_date, community_id, newsletter_t, subscription_count)
SELECT date(now()), a.community_id,
a.newsletter_type,
count(a.subscriber_user_id)
FROM
newsletter_subscribers_main a
LEFT OUTER JOIN nop_tbl b
ON (a.community_id = b.community_id)
AND (a.newsletter_type = b.newsletter_t)
AND (a.created_at = b.q_date)
WHERE b.q_date is null
AND b.mailing_list is null
GROUP BY a.community_id, a.newsletter_t, a.created_at

Answer 1

You have your explanation:
The objective of the query is to count subscriptions per (q_date, community_id, newsletter_t) in newsletter_subscribers_main and write the result to nop_tbl.
The LEFT JOIN prevents that rows are added multiple times.
But I also think, the query is inefficient and probably wrong.
The 2nd WHERE clause:
AND b.mailing_list is null
is just noise and can be removed. If b.q_date is null, then b.mailing_list is guaranteed to be null in this query.
You don't need parentheses around JOIN conditions.
If subscriber_user_id is defined NOT NULL, count(*) does the same, cheaper.
I suspect that grouping by a.created_at, while you insert date(now()) is probably wrong. Hardly makes any sense. My educated guess (assuming that created_at is type date):
INSERT INTO nop_tbl
(q_date, community_id, newsletter_t, subscription_count)
SELECT a.created_at
,a.community_id
,a.newsletter_type
,count(*)
FROM newsletter_subscribers_main a
LEFT JOIN nop_tbl b ON a.community_id = b.community_id
AND a.newsletter_type = b.newsletter_t
AND a.created_at = b.q_date
WHERE b.q_date IS NULL
GROUP BY a.created_at, a.community_id, a.newsletter_t;

Answer 2

The short short version is:
insert ... select ...
-> the query is filling nob_tbl
from ...
-> based on data in newsletter_subscribers_main
left join ... where ... is null
-> that are not already present in nob_tbl

Answer 3

Step by step
INSERT INTO nop_tbl
(q_date, community_id, newsletter_t, subscription_count)
The INSERT syntax This is telling the database in what table and what column will be used for the insert query
SELECT date(now()), a.community_id,
a.newsletter_type,
count(a.subscriber_user_id)
Those are instead the selected fields to insert
FROM
newsletter_subscribers_main a
Here is telling to the database to select fields which has alias prepended a. from table newsletter_subscribers_main
LEFT OUTER JOIN nop_tbl b
Here is left joining another table nop_tbl where other fields will be selected
ON (a.community_id = b.community_id)
AND (a.newsletter_type = b.newsletter_t)
AND (a.created_at = b.q_date)
Those are the rules of the JOIN, actually is telling what columns will be used for join
WHERE b.q_date is null
AND b.mailing_list is null
Those are the WHERE clauses , they are used to limit result to the requested data, in this case where two columns are null
GROUP BY a.community_id, a.newsletter_t, a.created_at
GROUP BY clauses, used to group result on given column
You can have a visual explanation of joins here