Write a mysql not in query as join - mysql

Question

I have two tables vtiger_crmentity and vtiger_crmentityrel (from open source project vtiger).
vtiger_crmentity
+--------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+-------+
| crmid | int(19) | NO | PRI | NULL | |
| smcreatorid | int(19) | NO | MUL | 0 | |
| smownerid | int(19) | NO | MUL | 0 | |
| modifiedby | int(19) | NO | MUL | 0 | |
| setype | varchar(30) | NO | | NULL | |
| description | text | YES | | NULL | |
| createdtime | datetime | NO | | NULL | |
| modifiedtime | datetime | NO | | NULL | |
| viewedtime | datetime | YES | | NULL | |
| status | varchar(50) | YES | | NULL | |
| version | int(19) | NO | | 0 | |
| presence | int(1) | YES | | 1 | |
| deleted | int(1) | NO | MUL | 0 | |
| label | varchar(255) | YES | MUL | NULL | |
+--------------+--------------+------+-----+---------+-------+
vtiger_crmentityrel
+-----------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+--------------+------+-----+---------+-------+
| crmid | int(11) | NO | | NULL | |
| module | varchar(100) | NO | | NULL | |
| relcrmid | int(11) | NO | | NULL | |
| relmodule | varchar(100) | NO | | NULL | |
+-----------+--------------+------+-----+---------+-------+
I am trying to get a list of contacts which are not present in the crmentityrel table (in the relcrmid column to be specific). I can do this via a subquery but it is taking about 2 minutes to complete (for about 20k records in each table).
I tried to convert the query to a join but i am surely doing something wrong as i keep getting wrong values (compared to the subquery which i know is right).
Any help is greatly appreciated. Please tell me if you need any details from my side
Edit -
My working query (with subquery) is -
SELECT crmid, label from vtiger_crmentity
WHERE deleted = 0 and setype="Contacts"
and crmid not in (select relcrmid from vtiger_crmentityrel
where relmodule="Contacts")

Answer 1

To convert a not in to a join, the idea is to use left join and where:
SELECT c.crmid, c.label
FROM vtiger_crmentity c left join
vtiger_crmentityrel cr
on c.crmid = cr.relcrmid and relmodule = 'Contacts'
WHERE c.deleted = 0 and c.setype = 'Contacts' and cr.relcrmid is null;
I should point out that the above is not exactly equivalent. NOT IN returns no rows if the subquery returns even a single NULL value. The above behaves more intuitively.
Because of the behavior of NOT IN with NULL values, NOT EXISTS is a better choice. Plus, it often has better performance as well:
SELECT crmid, label
FROM vtiger_crmentity c
WHERE deleted = 0 and setype = 'Contacts' AND
NOT EXISTS (SELECT relcrmid
FROM vtiger_crmentityrel cr
WHERE cr.relmodule = 'Contacts' and cr.relcrmid = c.crmid
);