Only allow one unique foreign key for primary - mysql

Question

I have 3 tables in mysql, One is a Company table, the other is a license table and the last is a joining table between both primary keys, When a person adds a company id to the license id in the joining table, it allows multiple companies to exist for one license, this cannot happen, so I need to do something that will only allow one company id for one license id
heres the tables
Table license
CREATE TABLE `License` (
`license_id` int(11) NOT NULL AUTO_INCREMENT,
`license_number` varchar(45) NOT NULL,
`start_date` date NOT NULL,
`end_date` date NOT NULL,
`duration` int(11) NOT NULL,
`expiry_date` date NOT NULL,
`product_id` int(11) DEFAULT NULL,
PRIMARY KEY (`license_id`)
) ENGINE=InnoDB AUTO_INCREMENT=21 DEFAULT CHARSET=latin1;
;
Company Table
CREATE TABLE `Company` (
`company_id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(255) NOT NULL,
`physical_address` varchar(255) DEFAULT NULL,
`postal_address` varchar(255) DEFAULT NULL,
`reseller_id` int(11) DEFAULT NULL,
PRIMARY KEY (`company_id`)
) ENGINE=InnoDB AUTO_INCREMENT=18 DEFAULT CHARSET=latin1;
and Joining table
CREATE TABLE `CompanyLicense` (
`license_id` int(11) NOT NULL,
`company_id` int(11) NOT NULL,
PRIMARY KEY (`license_id`,`company_id`),
KEY `companlicence_company_fk_idx` (`company_id`),
CONSTRAINT `companylicense_company_fk` FOREIGN KEY (`company_id`) REFERENCES `Company` (`company_id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `companylicense_license_fk` FOREIGN KEY (`license_id`) REFERENCES `License` (`license_id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
So far i have this
INSERT INTO CompanyLicense (license_id, company_id) VALUES
('2','6') on duplicate key update license_id = '2';
doesnt seem to do the job

Answer 1

You need to make company unique in companylicense:
ALTER TABLE companylicense ADD UNIQUE KEY (company)
or better yet, make company a field in license instead of having a link table.