I am trying to create a database to hold my address book but I've ran into a slight problem.
My database is supposed to hold individual contacts along with company contacts. That is an entry can be either for an individual or a company. As you know both individuals and companies have addresses and I want a One-To-Many relationship between individuals and addresses as well as companies and address.
So an individual can have many addresses
and a company can has many addresses
the problem is when trying to design the schema I am not sure what the best approach here is. Should I put two foreign keys in the address table, one for individual_Id and one for company_Id or should I create a linking table for each individual to address and company to address relationships.
The foreign keys do sound easy but don't know if that's the right approach, and linking seem like the right approach but don't know if this is overkill as they are mostly used for Many-To-Many relationships.
Thoughts?
There are various approaches to this...The approach we took was to have a "Names" table, the equivalent of your contacts table; an addresses table, and a phone numbers table.
The addresses and phone numbers tables each had a foreign key to the names table. The names table had a "type" field in it so that we could tell if the name was that of an individual, company, sole proprietor, etc.
The phone numbers table had a type field in it also, so we could tell whether the phone number was Work, Home, Cell, etc. In addition, we allowed the user to store a copy of one of the phone numbers in the Name table to serve as a Primary phone number.
Related
I have a table Subject
It has many fields, two of them are code and flag.
Earlier those two fields was an idempotention key for rows in this table.
But, now I need one more option system.
There are tens of rows in Subject
And 4-7 systems.
What is a better way?
Create table System for systems and create cross-table of mapping sysytems on subjects (code and flag are still in table Subject)
Create one table of mapping without creating table System
Just add another column in table Subject
Create table System and add to the table Subject foreign key for table System?
So, It's all about database normalization.
And the third option is pretty bad.
As for me the better way is fourth option.
But, I can`t explain to yourself why this option is better than 1 and 2.
So, I read rules of database normalization. And as for me, the first option satisfies all rules too.
This is the reason why I am asking this question.
It is not typically a great idea to design a SQL schema to 1st or 2nd normal form; many databases use something at or near 3rd normal form however there many still be some relationships in 3rd normal form where dependencies exist where redundancy still exists. This can be addressed by Boyce–Codd Normal Form (Codd, 1974).
It is also not typical to see 4th normal form and less so 5th normal form and beyond due to challenges with data maintenance in a "living" database.
Let's put this another way.
If you find yourself creating NULLable values constraints on many columns consider a table to contain those in an organized fashion - for example an Address table for addresses with a linking table from say for example a Person to a PersonAddress linking table to that Address table where the PersonAddress linking table might even have an AdddressTypeId column which links to an AddressType table with rows for Address Type Postal and Address Type Street or Address Type Business. For another example consider email addresses where people have personal, family, business and other email address types; even multiple of the same type for different uses; a doctor with a business practice email and 2-3 hospital email addresses where the doctor practices.
Linking tables for those type scenarios are likely better than 3-4 or more email or postal address columns in one table where many after the first are nullable or perhaps redundant.
Review your data; consider if your Subject for example may link to multiple System or placing a SubjectId column may lead to duplicates of that ID for differing system rows. If it is always and forever a 1-1 relationship it may be OK but for a 1-n or n-n it may not be ok to have the id in the other table and a linking table may provide a good mechanism to link them.
I am trying to figure out the best way to set up my Entity Diagram. I will explain based on the image below.
tblParentCustomer: This table stores information for our Primary Customers, which can either be a Business or Consumer.(They are identified using a lookup table tblCustomerType.)
tblChildCustomer: This table stores customers that are under the Primary Customer. The Primary Business customers can have Authorized Employees and Authorized Reps. The Primary Consumer customers can have Authorized Users. (They are identified using a lookup table tblCustomerType.)
tblChildAccountNumber: This table stores AccountNumbers for tblChildCustomer. These account numbers are mainly for the Child Business Customers. I may be adding Account Numbers for the Child Consumer customers, I am not sure yet, but I believe this design will allow for that if/when necessary.
Going back to tblParentCustomer : If this customer is a Consumer, I will need to add account numbers for them. My question is, do I create a 1 - Many relationship between tblParentCustomer and tblParentAccountNumber? This option would give me 2 different Account Number Tables.
Or would it make sense to create a Junction Account Table that intersects tblParentCustomer and tblChildCustomer?
The first option doesn't really make sense to me because what if there is only 1 Account number for a customer but multiple childCustomers?
Does it make sense to have 2 similar Account Tables that serve a different purpose?
Creating a many-to-many the way you want it to be, you need a link table that will make the whole thing go from 1-* and then *-1
That link table will have two FK, one linking to the parentTable and one linking to the childTable. Combination of those two FK will give you a composite PK (this is important to avoid duplicates). It will allow for any customer to be part of as many accounts as possible (duh.. it'll make the parent/child table a many-to-many relationship).
This approach is extremely common with regards to CRM or any Accounts containing people. Bring it one step further and in that table, you might want to add a "is primary contact" in the AccountMembers table. Drop the childAccountNumber table; you don't need it.
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
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.
I have been asked to add a new address book table to our database (SQL Server 2012).
To simplify the related part of the database, there are three tables each linked to each other in a one to many fashion: Company (has many) Products (has many) Projects and the idea is that one or many addresses will be able to exist at any one of these levels. The thinking is that in the front-end system, a user will be able to view and select specific addresses for the project they specify and more generic addresses relating to its parent product and company.
The issue now if how best to model this in the database.
I have thought of two possible ideas so far so wonder if anyone has had a similar type of relationship to model themselves and how they implemented it?
Idea one:
The new address table will additionally contain three fields: companyID, productID and projectID. These fields will be related to the relevant tables and be nullable to represent company and product level addresses. e.g. companyID 2, productID 1, projectID NULL is a product level address.
My issue with this is that I am storing the relationship information in the table so if a project is ever changed to be related to a different product, the data in this table will be incorrect. I could potentially NULL all but the level I am interested in but this will make getting parent addresses a little harder to get
Idea two:
On the address table have a typeID and a genericID. genericID could contain the IDs from the Company, Product and Project tables with the typeID determining which table it came from. I am a little stuck how to set up the necessary constraints to do this though and wonder if this is going to get tricky to deal with in the future
Many thanks,
I will suggest using Idea one and preventing Idea two.
Second Idea is called Polymorphic Association anti pattern
Objective: Reference Multiple Parents
Resulting side effect: Using dual-purpose foreign key will violating first normal form (atomic issue), loosing referential integrity
Solution: Simplify the Relationship
The simplification of the relationship could be obtained in two ways:
Having multiple null-able forging keys (idea number 1): That will be
simple and applicable if the tables(product, project,...) that using
the relation are limited. (think about when they grow up to more)
Another more generic solution will be using inheritance. Defining a
new entity as the base table for (product, project,...) to satisfy
Addressable. May naming it organization-unit be more rational. Primary key of this organization_unit table will be the primary key of (product, project,...). Other collections like Address, Image, Contract ... tables will have a relation to this base table.
It sounds like you could use Junction tables http://en.wikipedia.org/wiki/Junction_table.
They will give you the flexibility you need to maintain your foreign key restraints, as well as share addresses between levels or entities if that is desired.
One for Company_Address, Product_Address, and Project_Address
I just joined this site and this is my first question , I hope my question it's according to the StackOverflow question policy.
I'm designing a DB for Phone book which has the following abilities
Contact have 2 types (Company or Person)->ContactType
And I want each contact to have as many Emails, Phones Numbers, and Addresses as it wants.
And I want to specify which Person works in which Company , so I can show not only a Company Contact detail but also list of its employees and their jobs in that Company and their Contacts (CoEmpJob table)
I have designed a db diagram which is shown in the link below, is it well structured or can I achieve what I want in some better way?
Thanks in advance.
My Phone Book Design
As the design stands, you're missing a few things, such as a Companies table and a ContactTypes table. There seems to be no foreign key in the CoEmpJob table linking to the Contacts table.
In the Phones table, I personally wouldn't use a prefix field (unless you wish to display contacts by phone prefix), in which case every phone number is guaranteed to be unique, in which case the PhoneNum field becomes the primary key and the PhoneID field is unnecessary - but you might have the case in which husband and wife are in the same database; whilst they almost certainly have different mobile numbers, they almost certainly share the same home phone number! In this case, your design is correct.
I don't know how many people have more than one address (I would think very few, if at all) which means that the fields of the Address table could be moved into the Contacts table.
(Added)
As regards the companies, if you want to specify which Person works in which Company, then you will need a companies table (missing) and a join table (CoEmpJob). In the real world, this design would also require more tables - a join table can show which contacts are connected to which companies and what their current jobs are, but people change jobs (and companies) and so such a design would not store any history. Also, it is customary to link people (employees) to a department - and it is possible that one person can be connected to more than one department at a time, meaning that you will need another join table. This can get very complicated - it depends on what you want.
Your comment suggests that you want to store company data in the contacts table - this is a very bad idea; they should be kept separate.