I have problems in understanding why the following mapping from an ER-diagram to an ERM is correct, or, to be more precise complete. In the given example we have a ternary 1:1:N relationship between a project, a place and persons.
Each entity has a primary key (ProjectID, PlaceID, PersonID). If I understand this diagram correct the combination of a person and a project can not get associated to more than one place. And in addition the combination of a person and a place can only be associated with one project. Furthermore a project at a specific place can have multiple persons.
This understanding of how to read a ternary relationship leads to my problem. I map the ERM to the following RM:
Project(ProjectID)
Place(PlaceID)
Person(PersonID)
Works(ProjectID, PersonID, PlaceID)
I now have two candidate keys in the table Works: (Place, PersonID) and (ProjectID, PersonID). Let's choose the first one as primary key. I should then have a correct RM (literature told me) but what I don't get is how to ensure that the same combination of person and project is not associated with different places? Don't I have to say somewhere that (ProjectID, PersonID) is also a candidate key or is this not part of the RM notation?
ProjectID PersonID PlaceID
1 Marvin New York
1 Tom Paris
1 Marvin Tokyo
the combination of a person and a project can not get associated to more than one place. And in addition the combination of a person and a place can only be associated with one project.
Reading this carefully, you can make the further specification that every person has to be related to exactly one Project and exactly one Place. Since those are the only other entities in the relation, you can safely take Person (and more specifically PersonID) as the Primary Key for this relation.
Both (Place, PersonID) and (ProjectID, PersonID) are valid (non-primary) keys for the relation. You incorrectly assume that the example entries you give are correct. Seeing your relation, every personID, and by extension every (Place, PersonID) or (ProjectID, PersonID) pair, should appear only once in the table. So, the example table does not match the rules of the given ER-diagram.
Related
I have the graph with organizations and employees with 2 types of relationships:
(:Employee)-[:Worked]->(:Organization)
(:Employee)-[:Managed]->(:Organization)
Organization has unique property Id with index on it. Employee have property Name without index. I need to add new Employee to organization if he is not exist, or only add new relationship if he is exist. But i dont know how acheive this without index on Name.
1. Find organization by Id. Ok, this is fast.
match (o:Organization {Id:1})
2. Find employee that already linked to organization or add new link and employee if not exist. I cant use simple
merge (e:Employee {Name: "name"})
merge (e)-[:Worked]->(o)
because i dont have index on Name(this is will be slow) and i need find only employee that connected to selected organization
merge (e:Employee { Name: "name" })-[:Worked]->(o) doesnt work - it will add new employee if there is already exist employee with that name but other relationship, [:Managed] for example.
You seem to be misunderstanding the requirements for MERGE. An index is not required for MERGE to work.
In your specific case, having an index on :Employee(Name) is not required in order for MERGE (e:Employee {Name: "name"}) to work as expected. However, having such an index will speed up the processing of that MERGE clause, so it is recommended.
There are a few simple things to understand about how Neo4j handles Cypher MERGE operations to avoid undesired or unexpected behavior when using it.
The Cypher MERGE operation is a MATCH or CREATE of the entire pattern. This means that if any element of the pattern does NOT exist, Neo4j will attempt to create the entire pattern.
Always MERGE on each segment of the pattern that has the possibility to already exist.
After a MERGE operation you are guaranteed to have a useable reference to all identifiers established during the Cypher MERGE operation because they were either found or created.
Merge in CQL dosent require any index on the node. you can try the following :
merge (:Employee)-[:< relation_name >]->(:Organization)
If the nodes specified are existing, the relation among them is created else new nodes would be created and the relation among them is created .
I am trying to implement an entity relation for a hospital oracle database system.
I am rather confused if I should seperate the table below or merge them into 1.
- Supply
ItemNo (PK) , Name, ItemDescription, QuantityInStock, BackOrderLevel, CostPerUnit
- PharmaceuticalSupply
DrugNo (PK) , Dosage, MethodOfAdmin
Basically in my ERD, I pointed PharmaceuticalSupply to Supply as a subset which inherits the attribute but also have additional attributes. Am I wrong in doing that?
Ultimately, this is a design decision that has no right or wrong answer, but keeping them separate can be helpful. For example, there are many types of supplies that are not pharmaceutical. If you merge the tables, you make it possible to enter data that has no real meaning. For example, you can't have a dosage of bandages. The separate table makes it clear that dosage only applies to pharmaceuticals.
Note that there are a few variations on how to manage the PKs and FKs in PharmaceuticalSupply. It could have both an ItemNo and a DrugNo, where ItemNo is a foreign key. In that case, either one could be the primary key, but if DrugNo is the primary key, then ItemNo probably needs to be a unique index. However, unless DrugNo is needed due to some custom format, it might work well to simply use ItemNo as both PK and FK and completely eliminate DrugNo. This results in a "specialization" as the relational database world likes to refer to it.
It depends on your population. It it's a subset, to reduce redundancy add a foreign key to Supply. That way you'll be able to build a join that list all data.
I would still introduce a DrugNo key for indexing. Can an item number appear more than once in the PharmaceuticalSupply table ? If your do then your definitely need the DrugNo key.
PharmaceuticalSupply
DrugNo (PK) , ItemNo (FK), Dosage, MethodOfAdmin
I am Working on web application where i have 90 fields for a Person class which are divided in to family details,education details, personal details etc....
I want separate form for each, like for family details has-father name, mother name siblings etc... fields and so on for other
I want separate table for each detail with common reference id for all tables
My question is how many bean classes should i write? Is it with one bean class can i map from multiple forms to multiple tables?
class PersonRegister{
private Long iD;
private String emailID;
private String password;
.
.
}//for register.......
once logged in i need to maintain his/her details
Either
class person{
}
or
class PersonFamilyDetails{}
class PersonEducationDetails{}
etc
which way software developing standards specify to create?
Don't go overboard, I believe in your case single but very wide (i.e. with a lot of columns) table would be most efficient and simplest from maintenance perspective. Only thing to keep in mind is too query only for a necessary subset of columns/fields when loading lots of rows. Otherwise you'll be fetching kilobytes of unnecessary data, not needed for particular use case.
Unfortunately Hibernate doesn't have direct support for that, when designing a mapping for Person, you'll end up with huge class and even worse - Hibernate will always fetch all simple columns (and many-to-one relationships). You can however overcome this problem either by creating several views in the database containing only subset of columns or by having several Java classes mapping to the same table but only to subset of columns.
Splitting your database model into several tables is beneficial only if your schema is not normalized. E.g. when storing siblings first name and last name you may wish to have a separate Sibling table and next time some other family member is entered, you can reuse the same row. This makes database smaller and might be faster when searching by sibling.
Your question comes down to database normalization, as described in-depth by Boyce and Codd, see
http://en.wikipedia.org/wiki/Database_normalization.
The main advantage of database normalization is avoiding modification anomalies. In your case, if you got one table with for each person e.g. father-firstname and father-lastname, and you have multiple people with the same father, this data will be duplicated, and when you discover a typo in the father-lastname, you could modify it for one sibling, and not for the next.
In this simplified case, database design best practices would call for a first normalization into a separate table with father-id, father-firstname and father-lastname, and your person table having a one-to-many relation to it.
For one-to-one relations, e.g. person->personeducationdetails, there's some debate. In the original definition of 1st Normal Form, every optional field would be normalized by putting it's own table. This was later weakened by introducing 'null' in relational databases, see http://en.wikipedia.org/wiki/First_normal_form#cite_note-CoddRule-12. But still, if a whole set of columns could be null at the same time, you put them in a separate table with a one-to-one relation.
E.g. if you don't know a person's educationdetails, all of its related fields are null, so you better split them off in a separate table, and simply not have a personeducationdetails record for that person.
SQL Server 2012 MVC3 EF4.3.1 Code First project.
I have a Teacher and Student table with a one to many relationship. The Teacher’s tables Id will be used as the account number so its Id numbering needs to be separate from the Student’s. I would like to create a Person table (containing shared properties such as First, Last, Phone, Email) to reduce redundancy on the properties. Person will also have a one to many relationship to an Address table.
I’ve thought of trying a Table per Hierarchy model with Teacher and Student inheriting from Person but then the Id sets would not be separate and I would have to have a one to many relationship internally on the Person table. I could generate the ID’s through code but is an internal one to many doable or practical?
Another scenario would be to setup Person as a child table with a one to one between and Teacher and Person and a one to one between Student and Person but I’m not sure how or if it’s possible to have two separate one to one’s on a table.
Is there a practical way to do what I want or should I not worry about the redundancy and not use a Person table? If I went that route would it be possible to have two separate one to many relationships to an Address table (Teacher-Address and Student-Address)? Or for that matter a one to many (Teacher-Address, teacher may have an additional shipping address) and one to one (Student-Address)?
Thank you
Another way to do it is to have a one to one between a Person and a Role table. Teacher and Student are merely roles in this arrangement. A given Role can be fulfilled by many Person instances.
You could also do a Person table with an IsTeacher flag.
I can see two possibilities:
One: Go with your Student and Teacher inheriting from a base table of Person and not worry about the 'redundancy'. It's not a redundancy because your relating a Student and a Teacher not a Person to a Person and so in your database and DOM the Person table and Person class know nothing of the Teacher to Student relationship, it only knows that its a person. The teacher and student relationships are stored in there respective types, not the person type. Also, look at Table per Type instead of Table per Heiarchy. It's much cleaner and crisper looking in the database and you don't get all the information of each type in the heiarchy in one table.
Two: Create a table that specifically holds information that both Students and Teachers share and have that related to both the Student and Teacher table separately. You could call it something like "ContactInformation".
Being a teacher and being a student are roles of people, not types of people.
You should have a table for People, a table TeachCourse to say that a Person is the teacher of a course (which in some cases are multiple teachers), a table AssistCourse to say which persons are attending a class as a student. You might have people that teach a course and assist another course, and that wasn't properly modeled in your first version.
You can also create a ContactInformation or ShippingInformation table for People to specify all their data (Some people may have multiple phones, or emails to).
My application manages customer's complaints and has already been deployed into production. Each complaint has a code to identify it (for eaxmple "late delivery" ), a "department" type (wich is essentially the department responsible for that kind of complaint) and another "model" code which identifies the route through department's employees this complaint dossier has to follow (first to hr responsible then to hr big boss finally back to customer care). Each dossier has some common info and can have department specific infos, that's why i need deparment code.
For example Customer care get a complaint about "rudeness" of a call center operator, opens a dossier with code ABC and type "HR" (there's could be more HR dossier types). When the customer care has filled all the infos, forward it to hr(a mail is sent to the user configured in the system as HR responsible ). The hr employee fills his own section and send it back to customer care.
Till now each complaint code might have only one department and one model, now requirements have changed and i've two problems:
Some complaints are identified by the same code but might be due to different departments . For example a complaint about employees rudeness could be sent to the department which rules the call centers or to the department which rules logistics
i could solve this simply extending the table primary key to include the department (hoping they'll not decide the same code for the same department can follow different routes), changing application code might be a bit painful but it can be done :
Does extending primary keys to composite keys is a problem in Oracle or have side effects on existing records? the actual primary key is not used as foreign key anywere and all fields are filled.
this is a quite more difficult problem (at least for me): marketing department (the rulers) wants a special dossier.They monitor time departments take to answer complaints and open a new type of dossier if they exceeds the standard time.
For the above example, if hr always needs the 30% more time to complete employees rudeness dossiers, marketing can open an "inquire" dossier about that complaint code directed to hr.
Now, referring to point 1, i could add a new record for each complaint code having the second part of the key being the marketing code and associating it to a new model.This is going to double the rows of the table (which is already quite large). I see it very error prone for inserting new complaint codes.
I know it's very hard to give an opinion without being able to see the schema and the code, but i would appreciate your opinion anyway
"Does extending primary keys to
composite keys is a problem in Oracle
or have side effects on existing
records? the actual primary key is not
used as foreign key anywere and all
fields are filled."
Oracle allows us to have composite primary keys. They are not a problem from a relational perspective.
The only objection to primary composite keys is the usual one, that they make foreign key relationships and joins more cumbersome. You say you currently don't have foreign keys which reference this table. Nevertheless I would suggest you define a synthetic (surrogate) primary key using an index, and enforce the composite key as a unique constraint. Because you may well have foreign keys in the future: your very predicament shows that your current data model is not correct, or at least not complete.
"i could add a new record for each
complaint code having the second part
of the key being the marketing code"
Smart keys are dumb. Add a separate column for a marketing code if necessary. This would be populated if Marketing open their own dossier. I don't see why it needs to be associated with the Complaint Code or form part of any primary key (other than the Marketing Code lookup table).
I admit I don't fully understand your data model or business logic, so the following might be wrong. However what I think you want is a table DOSSIERS which can have two dossier types:
normal dossier identified by DEPT_CODE and COMPLAINT_CODE
Marketing dossier which I presume would be identified by DEPT_CODE, COMPLAINT_CODE and MARKETING_CODE.
Unique constraints permit NULL columns, so MARKETING_CODE can be optional. This is another advantage of using one instead of a composite primary key.
"I see it very error prone for
inserting new complaint codes."
Do you mean creating new complaints? Or new complaint types? Creating new complaints shouldn't be a problem: the process for creating Normal Dossiers will offer a choice of COMPLAINT_CODES where MARKETING_CODE is null, whereas the process for creating Marketing Dossiers will offer a choice of COMPLAINT_CODES where MARKETING_CODE is not null.
If you're talking about adding new complaint types then I suppose the question becomes: does there have to be a separate MARKETING_CODE for each regular COMPLAINT_CODE? I suspect not. In which case, instead of a MARKETING_CODE perhaps you need a CODE_TYPE - values NORMAL or MARKETING.