MYSQL Join three tables with exceptions - mysql

Question

I am working with three SQL tables and I am using INNER JOIN to join these tables. Here is an overview of what the tables look like.
CREATE TABLE `User` (
`id` varchar(32) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,
`firstName` varchar(64) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,
`lastName` varchar(64) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,
`email` varchar(64) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,
PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
CREATE TABLE `Ownership` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`user` varchar(32) COLLATE utf8_unicode_ci NOT NULL,
`certificate` int(10) unsigned NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `unicity` (`user`,`certificate`) ) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
CREATE TABLE `Certificate` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`domain` varchar(64) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,
`creationDate` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
`expirationDate` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
`type` varchar(32) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,
PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8
The request to join the tables is the following :
select user, lastName, domain
from Ownership
INNER JOIN User ON Ownership.user = User.id
INNER JOIN Certificate ON Ownership.certificate = Certificate.id;`
I have this type of result for example :
What I would like is to retrieve only one user for one domain name and create an exception to do the join only if the certificate.domain value does not already exists.
Is this something possible with MYSQL ?
Thank you

Answer 1

You need a pre-query to at least qualify some uniqueness component associated with a given domain. To get that, lets consider this.
select
C.domain,
min( O.user ) JustOneUser
from
Certificate C
JOIN Ownership O
on C.ID = O.Certificate
group by
C.Domain
So this will get one user for a given domain. Now, use THAT result to get the user associated to the domain.
select
PQ.Domain,
U.LastName,
U.FirstName
from
(select
C.domain,
min( O.user ) JustOneUser
from
Certificate C
JOIN Ownership O
on C.ID = O.Certificate
group by
C.Domain ) PQ
JOIN User U
on PQ.JustOneUser = U.ID