I have designed a simple database ER Diagram, now I've to normalize it. I've a ternary operation in my ER Diagram they are:
Staff(StaffId,StaffName);
Student(StudentId,StudentName);
Opportunity(OppId{PK})
The relationship_name of these three is responsible and cardinality is Many-to-Many.
My question is what do first in normalizing , is it many- to -many relationship or the ternary relationship ?
Related
I'm learning about creating databases in MySQL and one of the theoretical parts is developing ER diagram.
Do I really need it when making my own project? And if I want to create one, do I create it after normalization of relations?
1). You're not required to do it. But it can definitely help to keep a clear overview over your scheme.
2). I'd just start by making an ER diagram and updating it after, or during, normalization. You could use tools like MySql Workbench to easily make and manage ER diagrams
Normalization technique must be the part of database refinement which has to be carried out before Entity Relationship Diagram.
As in ERD technique, we find out Primary Key and Foreign Key, using the same relationship amongst the entities.
I've made a ER Diagram using the Chen Notation and using a casus. Could anyone check if I'm doing it the right way?
Besides this I have some other questions:
Why does the weak entity always has one and one only? (thick arrow form assignments to has)
How do I process the salary between 20 times the scale and 35 times the scale in a ER Diagram?
I hope someone can help me
The ER Diagram I've made so far
http://imgur.com/ejIGO7S
ER diagram means Entity-Relationship diagram.
you can not write there about the process on Attributes.
you can do salary's scaling regarding a process in a query.
is there any way in MySQL or tool that could help me to transform a ER model to Logical model?
The ER model was done in MySQL, so if there's a way to use the file extension in another software that would be wonderful.
MySQL Workbench's models aren't really ER models. They're physical models consisting of tables and foreign key constraints. The entity-relationship model distinguishes between entity relations and relationship relations, but these would both be represented as tables in MySQL Workbench. Relationships in ER don't map to foreign key constraints, rather FK constraints enforce the domains of roles in relationships.
Automated translation from a physical to a logical model isn't possible, since the physical model doesn't capture domains or functional and join dependencies. ER models fare slightly better - domains are represented and some dependencies, but ER can't represent all possible relationships and constraints.
Entity-Relationship is confusing me. In some examples I couldn't find relationship between some entities, in other words I can't track information between them.
My question is
"Is it necessary to have relationship between all entities in database. In other words, is it ok to have diagram without relationship, whether there is no relation, whether for simplicity"
Thanks in advance
Logically, entities are implicitly related by attributes having the same domain, and a database can be understood as a set of related data. Thus, on the logical level, I believe that all entities in a database must necessarily be related.
On an ER diagram, however, the absence of relationship lines make it difficult to determine the foreign key domains, cardinality and constraints. It's bad enough that most ER diagrams show physical types and not domains, and that most are poorly normalized. Without any hint of dependencies, all we have is a visual table dump.
Where there is no direct relationship between two entities, no indication is required. It's fine to show subsets of a schema in a diagram, to focus on particular aspects, as long as the missing information is indicated elsewhere. Leaving out important and necessary information is not acceptable, though.
Are there generally accepted computer science terms for one-to-one and one-to-many relationships? Something similar to the terms "aggregation", "generalization", and "association," but strictly pertaining to the cardinality of the relationship?
From everything I've ever read, one-to-one, one-to-many, etc. are the accepted ways of referring to these relationships. They are very self descriptive of what they're about.