ER diagram that implements a database for trainee - database

Question

I edited and remade the ERD. I have a few more questions.
I included participation constraints(between trainee and tutor), cardinality constraints(M means many), weak entities (double line rectangles), weak relationships(double line diamonds), composed attributes, derived attributes (white space with lines circle), and primary keys.
Questions:
Apparently to reduce redundant attributes I should only keep primary keys and descriptive attributes and the other attributes I will remove for simplicity reasons. Which attributes would be redundant in this case? I am thinking start_date, end_date, phone number, and address but that depends on the entity set right? For example the attribute address would be removed from Trainee because we don't really need it?
For the part: "For each trainee we like to store (if any) also previous companies (employers) where they worked, periods of employment: start date and end date."
Isn't "periods of employment: start date, end date" a composed attribute? because the dates are shown with the symbol ":" Also I believe I didn't make an attribute for "where they worked" which is location?
Also how is it possible to show previous companies (employers) when we already have an attribute employers and different start date? Because if you look at the Question Information it states start_date for employer twice and the second time it says start_date and end_date.
I labeled many attributes as primary keys but how am I able to distinguish from derived attribute, primary key, and which attribute would be redundant?
Is there a multivalued attribute in this ERD? Would salary and job held be a multivalued attribute because a employer has many salaries and jobs.
I believe I did the participation constraints (there is one) and cardinality constraints correctly. But there are sentences where for example "An instructor teaches at least a course. Each course is taught by only one instructor"; how can I write the cardinality constraint for this when I don't have a relationship between course and instructor?
Do my relationship names make sense because all I see is "has" maybe I am not correctly naming the actions of the relationships? Also I believe schedules depend on the actual entity so they are weak entities.... so does that make course entity set also a weak entity (I did not label it as weak here)?
For the company address I put a composed attribute, street num, street address, city... would that be correct? Also would street num and street address be primary keys?
Also I added the final mark attribute to courses and course_schedule is this in the right entity set? The statement for this attribute is "Each trainee identified by: unique code, social security number, name, address, a unique telephone number, the courses attended and the final mark for each course."
For this part: "We store in the database all classrooms available on the site" do i make a composed attribute that contains site information?
Question Information:
A trainee may be self-employed or employee in a company
Each trainee identified by:
unique code, social security number, name, address, a unique
telephone number, the courses attended and the final mark for each course.
If the trainee is an employee in a company: store the current company (employer), start date.
For each trainee we like to store (if any) also previous companies (employers) where they worked, periods of employment: start date and end date.
If a trainee is self-employed: store the area of expertise, and title.
For a trainee that works for a company: we store the salary and job
For each company (employer): name (unique), the address, a unique telephone number.
We store in the database all known companies in the
city.
We need also to represent the courses that each trainee is attending.
Each course has a unique code and a title.
For each course we have to store: the classrooms, dates, and times (start time, and duration in minutes) the course is held.
A classroom is characterized by a building name and a room number and the maximum places’ number.
A course is given in at least a classroom, and may be scheduled in many classrooms.
We store in the database all classrooms
available on the site.
We store in the database all courses given at least once in the company.
For each instructor we will store: the social security number, name, and birth date.
An instructor teaches at least a course.
Each course is taught by only one instructor.
All the instructors’ telephone numbers must also be stored (each instructor has at least a telephone number).
A trainee can be a tutor for one or many trainees for a specific
period of time (start date and end date).
For a trainee it is not mandatory to be a tutor, but it is mandatory to have a tutor

Answer 1

The attribute ‘Code’ will be your PK because it’s only use seems to be that of a Unique Identifier.
The relationship ‘is’ will work but having a reference to two tables like that can get messy. Also you have the reference to "Employers" in the Trainee table which is not good practice. They should really be combined. See my helpful hints section to see how to clean that up.
Company looks like the complete table of Companies in the area as your details suggest. This would mean table is fairly static and used as a reference in your other tables. This means that the attribute ‘employer’ in Employed would simply be a Foreign Key reference to the PK of a specific company in Company. You should draw a relationship between those two.
It seems as though when an employee is ‘employed’ they are either an Employee of a company or self-employed.
The address field in Company will be a unique address your current city, yes, as the question states the table is a complete list of companies in the city. However because this is a unique attribute you must have specifics like street address because simply adding the city name will mean all companies will have the same address which is forbidden in an unique field.
Some other helpful hints:
Stay away from adding fields with plurals on them to your diagram. When you have a plural field it often means you need a separate table with a Foreign Key reference to that table. For example in your Table Trainee, you have ‘Employers’. That should be a Employer table with a foreign key reference to the Trainee Code attribute. In the Employer Table you can combine the Self-employed and Employed tables so that there is a single reference from Trainee to Employer.
ERD Link http://www.imagesup.net/?di=1014217878605. Here's a quick ERD I created for you. Note the use of linker tables to prevent Many to Many relationships in the table. It's important to note there are several ways to solve this schema problem but this is just as I saw your problem laid out. The design is intended to help with normalization of the db. That is prevent redundant data in the DB. Hope this helps. Let me know if you need more clarification on the design I provided. It should be fairly self explanatory when comparing your design parameters to it.
Follow Up Questions:
If you are looking to reduce attributes that might be arbitrary perhaps phone_number and address may be ones to eliminate, but start and end dates are good for sorting and archival reasons when determining whether an entry is current or a past record.
Yes, periods_of_employment does not need to be stored as you can derive that information with start and end dates. Where they worked I believe is just meant to say previous employers, so no location but instead it’s meant that you should be able to get a list all the employers the trainee has had. You can get that with the current schema if you query the employer table for all records where trainee code equals requested trainee and sort by start date. The reason it states start_date twice is to let you know that for all ‘previous’ employers the record will have a start and end date. Hence the previous. However, for current employers the employment hasn't ended which means there will be no end_date so it will null. That’s what the problem was stating in my opinion.
To keep it simple PK’s are unique values used to reference a record within another table. Redundant values are values that you essentially don’t need in a table because the same value can be derived by querying another table. In this case most of your attributes are fine except for Final_Mark in the Course table. This is redundant because Course_Schedule will store the Final_Mark that was received. The Course table is meant to simply hold a list of all potential courses to be referenced by Course_Schedule.
There is no multivalued attributes in this design because that is bad practice Job and salary are singular and if and job or salary changes you would add a new record to the employer table not add to that column. Multivalued attributes make querying a db difficult and I would advise against it. That’s why I mentioned earlier to abstract all attributes with plurals into their own tables and use a foreign key reference.
You essentially do have that written here because Course_Schedule is a linker table meaning that it is meant to simplify relationships between tables so you don’t have many to many relationships.
All your relationships look right to me. Also since the schedules are linker tables and cannot exist without the supporting tables you could consider them weak entities. Course in this schema is a defined list of all courses available so can be independent of any other table. This by definition is not a weak entity. When creating this db you’d probably fill in the course table and it probably wouldn’t change after that, except rarely when adding or removing an available course option.
Yes, you can make address a composite attribute, and that would be right in your diagram. To be clear with your use of Primary key, just because an attribute is unique doesn’t make it a primary key. A table can have one and only one primary key so you must pick a column that you are certain will not be repeated. In this example you may think street number might be unique but what if one company leaves an address and another company moves into that spot. That would break that tables primary key. Typically a company name is licensed in a city or state so cannot be repeated. That would be a better choice for your primary key. You can also make composite primary keys, but that is a more advanced topic that I would recommend reading about at a later date.
Take final_mark out of courses. That’s table will contain rows of only courses, those courses won’t be linked to any trainee except by course_schedule table. The Final_Mark should only be in that table. If you add final_mark to Course table then, if you have 10 trainees in a course, You’d have 10 duplicate rows in the course table with only differing final_marks. Instead only hold the course_code and title that way you can assign different instructors, trainees and classrooms using the linker tables.
No composite attribute is needed using this schema. You have a Classroom table that will hold all available classrooms and their relevant information. You then use the Classroom_Schedule linker table to assign a given Classroom to a Course_Schedule. No attributes of Classroom can be broken down to simpler attributes.