Is the use of cursor needed to achieve the following query? - mysql

Question

Consider the database:
A Company has zero or more Branch companies
A Company has zero or more Emails
A Branch has zero or more Emails
And consider the following instance of that database:
1 Company with 2 Branches
The Company has 1 Email
Each branch has 2 Emails
(5 Emails in total)
If you prefer, check the graphical representation of that instance.
Query
I'd like to make a query (MySQL) that retrieves all e-mails related to a given company (5, in the example). I couldn't find a solution, so after a bit of googling I found out that there's a thing called cursor.
Question
Is the use of cursor needed to achieve the query above? Would it be a good solution? What would be your solution to the query above?
(tables are defined below)
My idea was to retrieve all branches for a given company (that query I could make), but from there I don't know how to iterate through the branches (result of the query) and get all the e-mails.
Tables definitions in MySQL
-- Database tables
CREATE TABLE `company` (
`c_id` INT(11) NOT NULL AUTO_INCREMENT,
`c_name` VARCHAR(45) NOT NULL,
PRIMARY KEY (`c_name`),
UNIQUE KEY `c_id` (`c_id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8
CREATE TABLE `branch` (
`b_id` INT(11) NOT NULL AUTO_INCREMENT,
`c_id` INT(11) NOT NULL,
`b_name` VARCHAR(45) NOT NULL,
PRIMARY KEY (`b_name`),
UNIQUE KEY `b_id` (`b_id`),
KEY `c_id` (`c_id`),
CONSTRAINT `branch_ibfk_1` FOREIGN KEY (`c_id`)
REFERENCES `company` (`c_id`)
ON DELETE CASCADE
ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8
CREATE TABLE `email` (
`e_id` INT(11) NOT NULL AUTO_INCREMENT,
`e_addr` VARCHAR(45) NOT NULL,
PRIMARY KEY (`e_addr`),
UNIQUE KEY `e_id` (`e_id`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8
CREATE TABLE `c_email` (
`c_id` INT(11) NOT NULL,
`e_id` INT(11) NOT NULL,
PRIMARY KEY (`c_id`,`e_id`),
KEY `e_id` (`e_id`),
CONSTRAINT `c_email_ibfk_1` FOREIGN KEY (`c_id`)
REFERENCES `company` (`c_id`)
ON DELETE CASCADE
ON UPDATE CASCADE,
CONSTRAINT `c_email_ibfk_2` FOREIGN KEY (`e_id`)
REFERENCES `email` (`e_id`)
ON DELETE CASCADE
ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8
CREATE TABLE `b_email` (
`b_id` INT(11) NOT NULL,
`e_id` INT(11) NOT NULL,
PRIMARY KEY (`b_id`,`e_id`),
KEY `e_id` (`e_id`),
CONSTRAINT `b_email_ibfk_1` FOREIGN KEY (`b_id`)
REFERENCES `branch` (`b_id`)
ON DELETE CASCADE
ON UPDATE CASCADE,
CONSTRAINT `b_email_ibfk_2` FOREIGN KEY (`e_id`)
REFERENCES `email` (`e_id`)
ON DELETE CASCADE
ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8
-- Example of query
-- Retrieving all e-mails of a given branch
SELECT e_addr
FROM email AS _em
JOIN b_email AS _be ON _be.e_id = _em.e_id
JOIN branch AS _br ON _br.b_id = _be.b_id
WHERE b_name = #bra2;
-- How to retrieve all e-mails related to a given company
-- (e-mails from the company itself and also from all of its branches)?
-- ?

Answer 1

No, you don't need a cursor for this. You can do this with SQL -- much better than a cursor. It is something like:
select *
from ((select ce.c_id, e.email
from c_email ce join
email e
on e.e_id = ce.e_id
) union all
(select b.c_id, e.email
from branch b join
b_email be
on b.b_id = be.b_id join
email e
on be.e_id = e.e_id
)
) e
where ce.c_id = X;
EDIT:
You can add b_id into the second subquery but not the first. Perhaps you could do:
select *
from ((select ce.c_id, e.email, NULL as b_id
from c_email ce join
email e
on e.e_id = ce.e_id
) union all
(select b.c_id, e.email, b.b_id
from branch b join
b_email be
on b.b_id = be.b_id join
email e
on be.e_id = e.e_id
)
) e
where ce.c_id = X
order by c_id, b_id;