SQL Relationship Cardinality in Logical Design - sql

Question

I have the following issue. We are currently working on a system for a shuttle service company. Now, part of the entities in the database for this system include numerous lookup tables (such as vehicle_type, employee_status, etc), as well as some other tables, such as vehicle and vehicle_service log.
Now the issue we as a team are having is that we cannot agree on what the logical relationship cardinalities between entities should be. The two main problem relationships include the tables defined as follows:
CREATE TABLE IF NOT EXISTS `user_type` (
`type_id` tinyint(4) NOT NULL AUTO_INCREMENT,
`description` varchar(200) NOT NULL,
PRIMARY KEY (`type_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COMMENT='Store the user types - employee
or consultant' AUTO_INCREMENT=1 ;
which is linked to
CREATE TABLE IF NOT EXISTS `user` (
`user_id` int(11) NOT NULL AUTO_INCREMENT,
`username` varchar(100) NOT NULL,
`password` varchar(100) NOT NULL,
`user_type` tinyint(4) NOT NULL,
PRIMARY KEY (`user_id`),
KEY `user_type` (`user_type`),
KEY `username` (`username`),
KEY `login` (`username`,`password`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COMMENT='Table used when logging in
to check access level, type of user, etc. ' AUTO_INCREMENT=1 ;
The user table includes other irrelevant data. So the issue here is that I say (because MySQL Workbench reverse engineered it that way and it makes more sense) that the relationship should be 1-many, while another team member says that it should be 0-many (because some records may exist in the user_type table that aren't used in the user table)
The other table relationship we are having words about are defined as follows:
CREATE TABLE IF NOT EXISTS `vehicle` (
`vehicle_id` int(11) NOT NULL AUTO_INCREMENT,
`registration_number` varchar(10) NOT NULL,
PRIMARY KEY (`vehicle_id`),
UNIQUE KEY `registration_number` (`registration_number`),
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COMMENT='Actual vehicle information'
AUTO_INCREMENT=1 ;
Again, with some other columns not relative to the question. This links with
CREATE TABLE IF NOT EXISTS `service_log` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`vehicle_id` int(11) NOT NULL,
`description` text NOT NULL,
`date` date NOT NULL,
`cost` double NOT NULL,
PRIMARY KEY (`id`),
KEY `vehicle_id` (`vehicle_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COMMENT='Store records of all services
to vehicles' AUTO_INCREMENT=1 ;
Should this be 1-many or 0-many because a vehicle may not yet go in for a service? According to me it should be 1-many, but I don't know if this works logically.
We are all very confused about this whole logical modelling thing, so any help would be much appreciated!
I figured it would be easier for me to create the DB first and then reverse engineer it to a physical model, but never though about logical.

Answer 1

Zero to many if it is optional. Say for example a Sales Rep would have a zero or many customer. Why is that? Because if there is a new sales rep then it would mean he/she has no customer to begin with unless of course he/she assume the accounts of a resigned Sales Rep.
On the other hand one or many is mandatory. For example a an Order which has order date and customer who ordered should have at least one record on Order Detail table. Let's say a customer ordered a tablet last 04/22/2013 then he/she would have:
Order table
----------------------------------------
Orderid. OrderDate. Customermnum
----------------------------------------
1. 04/22/2013 101
Order detail table
----------------------------------------
Orderid. Productid. Qty. quotedprice
----------------------------------------
1. T101 1 500
So, in your case User to UserType is 1 to 0 or many beacause a user type may have not been used by any user yet.
Now, vehicle to service It is also 1 to 0 or many since a vehicle may not necessarily have a service done yet.