CODEWITHSUNDEEP
mysqlsqljoinsubquery
Alex Giotis
Alex Giotis

Reputation: 611

How to rewrite a NOT IN subquery as join

Let's assume that the following tables in MySQL describe documents contained in folders. 

mysql> select * from folder;
+----+----------------+
| ID | PATH           |
+----+----------------+
|  1 | matches/1      |
|  2 | matches/2      |
|  3 | shared/3       |
|  4 | no/match/4     |
|  5 | unreferenced/5 |
+----+----------------+


mysql> select * from DOC;
+----+------+------------+
| ID | F_ID | DATE       |
+----+------+------------+
|  1 |    1 | 2000-01-01 |
|  2 |    2 | 2000-01-02 |
|  3 |    2 | 2000-01-03 |
|  4 |    3 | 2000-01-04 |
|  5 |    3 | 2000-01-05 |
|  6 |    3 | 2000-01-06 |
|  7 |    4 | 2000-01-07 |
|  8 |    4 | 2000-01-08 |
|  9 |    4 | 2000-01-09 |
| 10 |    4 | 2000-01-10 |
+----+------+------------+

The columns ID are the primary keys and the column F_ID of table DOC is a not-null foreign key that references the primary key of table FOLDER. By using the 'DATE' of documents in the where clause, I would like to find which folders contain only the selected documents. For documents earlier than 2000-01-05, this could be written as:

SELECT DISTINCT d1.F_ID 
FROM DOC d1 
WHERE d1.DATE < '2000-01-05' 
AND d1.F_ID NOT IN (
    SELECT d2.F_ID 
    FROM DOC d2 WHERE NOT (d2.DATE < '2000-01-05')
);

and it correctly returns '1' and '2'. By reading http://dev.mysql.com/doc/refman/5.5/en/rewriting-subqueries.html the performance for big tables could be improved if the subquery is replaced with a join. I already found questions related to NOT IN and JOINS but not exactly what I was looking for. So, any ideas of how this could be written with joins ?

Upvotes: 3

Views: 8669

Answers (4)

Mani
Mani

Reputation: 634

Sample Table and Insert Statements

CREATE TABLE `tleft` (
  `id` int(2) NOT NULL,
  `name` varchar(100) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

CREATE TABLE `tright` (
  `id` int(2) NOT NULL,
  `t_left_id` int(2) DEFAULT NULL,
  `description` varchar(100) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8



INSERT INTO `tleft` (`id`, `name`)
VALUES
    (1, 'henry'),
    (2, 'steve'),
    (3, 'jeff'),
    (4, 'richards'),
    (5, 'elon');



INSERT INTO `tright` (`id`, `t_left_id`, `description`)
VALUES
    (1, 1, 'sample'),
    (2, 2, 'sample');

Left Join : SELECT l.id,l.name FROM tleft l LEFT JOIN tright r ON l.id = r.t_left_id ;

Returns Id : 1, 2, 3, 4, 5

Right Join : SELECT l.id,l.name FROM tleft l RIGHT JOIN tright r ON l.id = r.t_left_id ;

Returns Id : 1,2

Subquery Not in tright : select id from tleft where id not in ( select t_left_id from tright);

Returns Id : 3,4,5

Equivalent Join For above subquery : 

SELECT l.id,l.name FROM tleft l LEFT JOIN tright r ON l.id = r.t_left_id WHERE r.t_left_id IS NULL;

AND clause will be applied during the JOIN and WHERE clause will be applied after the JOIN .

Example : SELECT l.id,l.name FROM tleft l LEFT JOIN tright r ON l.id = r.t_left_id AND r.description ='hello' WHERE r.t_left_id IS NULL ;

Hope this helps

Upvotes: 0

JustDanyul
JustDanyul

Reputation: 14044

If I understand your question correctly, that you want to find the F_IDs representing folders which only contains documents from before '2000-01-05', then simply

SELECT F_ID 
FROM DOC 
GROUP BY F_ID 
HAVING MAX(DATE) < '2000-01-05'

Upvotes: 1

Gordon Linoff
Gordon Linoff

Reputation: 1269803

The general answer is:

select t.*
from t
where t.id not in (select id from s)

Can be rewritten as:

select t.*
from t left outer join
     (select distinct id from s) s
     on t.id = s.id
where s.id is null

I think you can apply this to your situation.

Upvotes: 11

D&#39;Arcy Rittich
D&#39;Arcy Rittich

Reputation: 171411

select distinct d1.F_ID
from DOC d1
left outer join (
    select F_ID
    from DOC 
    where date >= '2000-01-05'
) d2 on d1.F_ID = d2.F_ID
where d1.date < '2000-01-05' 
    and d2.F_ID is null

Upvotes: 4

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