can 2 entities have 2 relationships - mysql

Question

In database design, can 2 entities have 2 relationships among themselves? i.e for example there are 2 entities donor and admin.. there are 2 relationships
1. admin accesses donor details
2. admin can contact donor and vice versa
can we join them with 2 relationships?

Answer 1

Definitely, although how much sense it makes to model "accesses" and "contacts" relations in a database depends on your application. I'll stay with your example though and assume these relations are n to n. Here is how the SQL could look like (warning, syntax not tested):
CREATE TABLE admin (
id int unsigned AUTO_INCREMENT PRIMARY KEY,
name text NOT NULL
);
CREATE TABLE donor (
id int unsigned AUTO_INCREMENT PRIMARY KEY,
name text NOT NULL
);
CREATE TABLE admin_donor_access_details (
id_admin int unsigned NOT NULL,
id_donor int unsigned NOT NULL,
PRIMARY KEY (id_admin, id_donor),
CONSTRAINT FOREIGN KEY(id_admin) REFERENCES admin(id) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT FOREIGN KEY(id_donor) REFERENCES donor(id) ON DELETE CASCADE ON UPDATE CASCADE
);
CREATE TABLE admin_donor_contact (
id_admin int unsigned NOT NULL,
id_donor int unsigned NOT NULL,
PRIMARY KEY (id_admin, id_donor),
CONSTRAINT FOREIGN KEY(id_admin) REFERENCES admin(id) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT FOREIGN KEY(id_donor) REFERENCES donor(id) ON DELETE CASCADE ON UPDATE CASCADE
);
The two relations could also be expressed in a single join table with boolean flags, like this:
CREATE TABLE admin_donor (
id_admin int unsigned NOT NULL,
id_donor int unsigned NOT NULL,
detail_access tinyint(1) NOT NULL,
contact tinyint(1) NOT NULL,
PRIMARY KEY (id_admin, id_donor),
CONSTRAINT FOREIGN KEY(id_admin) REFERENCES admin(id) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT FOREIGN KEY(id_donor) REFERENCES donor(id) ON DELETE CASCADE ON UPDATE CASCADE
);
This will put some extra effort on your code because you need to determine whether to insert or update a row when adding a relationship, and whether to delete or update a row when removing a relationship, but in my opinion this is still a usable alternative.