Best normalised practice to map phone numbers with user or office - mysql

Question

I am designing a DB in MySQL. I have a user table, office table and a phone number table as I intend to allow multiple phone numbers to a user or for an office. so I know I need to add foreign keys in phone number table to office table and to user table rather than the other way round. But how do I do it without a null record?
What is the best-normalized way to make this with neither officeId not userId in the phone number record null?
MY Phone number table
CREATE TABLE `phonenumber` (
`Id` varchar(36) NOT NULL,
`Title` varchar(100) DEFAULT NULL,
`UserId` varchar(36) DEFAULT NULL,
`SchoolId` int(11) DEFAULT NULL,
`CreatedOn` datetime NOT NULL,
`ModifiedOn` datetime NOT NULL,
`CreatedBy` varchar(36) NOT NULL,
`ModifiedBy` varchar(36) NOT NULL,
`Number` varchar(20) NOT NULL,
PRIMARY KEY (`Id`),
KEY `FK_Phone_User` (`UserId`),
KEY `FK_Phone_School` (`SchoolId`),
CONSTRAINT `FK_Phone_School` FOREIGN KEY (`SchoolId`) REFERENCES `school` (`Id`),
CONSTRAINT `FK_Phone_User` FOREIGN KEY (`UserId`) REFERENCES `user` (`Id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
My User Table
CREATE TABLE `user` (
`Id` varchar(36) NOT NULL,
`UserName` varchar(20) NOT NULL,
`DisplayName` varchar(100) DEFAULT NULL,
`OfficialName` varchar(100) DEFAULT NULL,
`PasswordHash` varchar(50) DEFAULT NULL,
`CreatedOn` datetime DEFAULT NULL,
`CreatedBy` varchar(36) DEFAULT NULL,
`ModifiedOn` datetime DEFAULT NULL,
`ModifiedBy` varchar(36) DEFAULT NULL,
PRIMARY KEY (`Id`),
UNIQUE KEY `UserName` (`UserName`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
My School table
CREATE TABLE `school` (
`Id` int(11) NOT NULL AUTO_INCREMENT,
`Name` varchar(100) NOT NULL,
`AddressId` varchar(36) DEFAULT NULL,
`CreatedOn` datetime NOT NULL,
`ModifiedOn` datetime NOT NULL,
`CreatedBy` varchar(36) NOT NULL,
`ModifiedBy` varchar(36) NOT NULL,
PRIMARY KEY (`Id`),
KEY `FK_School_Address` (`AddressId`),
CONSTRAINT `FK_School_Address` FOREIGN KEY (`AddressId`) REFERENCES `address` (`Id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Answer 1

You simply have to allow for NULL fields during the creation of the Phone_numbers table. What you were warned against is having a whole record with NULLs. You are correctly handling the field that but not saying anything or saying NULL. Here's an example:
CREATE TABLE Phone_numbers(
phone_number VARCHAR(15) NOT NULL,
user_id INT UNSIGNED,
office_id INT UNSIGNED,
PRIMARY KEY (phone_number),
CONSTRAINT RefUsers1 FOREIGN KEY (user_id)
REFERENCES Users(user_id),
CONSTRAINT RefOffices2 FOREIGN KEY (office_id)
REFERENCES Offices(office_id)
)ENGINE=INNODB
;
For this simple model