My assignment requires that I have an ER diagram that shows the dependencies between the objects in my solution. Mine consists of several procedures, tables, a trigger and a sequence. Does anyone know if these objects are permitted in a SQL ER diagram? I ask this because the example shown by my lecturer consisted only of tables. If these objects are allowed in an ER diagram, what is the proper way to represent them?
No, an ER diagram should not contain procedures, triggers nor sequences.
An Entity-Relashioship Diagram is used to represent the relashionships between entities in a database. Procedures, triggers and sequences do not contribute to the relashionship representations.
ER Diagram don't have any particular representation of triggers because ER-Diagram is logical design of Database. BUT we use Strong entity and weak entity symbols to show the dependence of tablesStrong Weak
You should use Entities (the squares), Relationships (the diamonds) and attributes (the ellipsis). All of them are connected by simple lines.
Finally, and most important. Entities and Relationships are not Tables, as you say. Those are part of a physical model.
Related
I have recently learned about “barred relationships” when developing an ERD through the Oracle Academy service. The course gave the following definition of a barred relationship: “A relationship that participates in an entity’s unique identifier” I understand how this works in the context of resolving a M:M relationship by creating an intersection entity and having the UID of that entity come from the originating relationships. My question is: Can a barred relationship occur in an instance where there is no intersection entity, or are barred relationships reserved for intersection entities?
Barred, that is, bar-ed ids are literally the original ids of the two entities connected by the relation with a bar (pipe, |) in between. Like in a CSV file.
The term is unusual and an invention of that course, I would say. It certainly is not usually used in the generic SQL world, nor even in the Oracle RDBMS documentation.
Below is the ER diagram with relation among 5 different entities.
My question:
1)
Each of these 5 entities a class as per OOP terminology?
2)
Can you help me understand the meaning of relation(different types of lines) among these 5 entities? Line with a little bubble/ with an angular shape /text
This is not an UML diagram at all, but an entity-relationship (E-R) diagram and typically shows the DB design. You can easily google it to find the detailed notstional rules, I could quickly find this page:
http://www.google.de/imgres?imgurl=http://docs.oracle.com/cd/B12037_01/java.101/b12021/img/entity_d.gif&imgrefurl=http://docs.oracle.com/cd/B12037_01/java.101/b12021/dev.htm&h=1780&w=1556&tbnid=vbXfAtrAAIq5_M:&zoom=1&tbnh=97&tbnw=85&usg=__vyr0LMggQHqYtI8Q6Ix-722jJwg=&docid=3LbfUSlxwqfpMM&client=firefox-a&sa=X&ei=iX1HU5awEImN7Qbf6IGwDg&ved=0CEsQ9QEwBA&dur=1439
If you want to model DB in UML, you can still do it, using class diagram and eventually restricting permited relationships to those available in non-OO modelling.
And in addition to that, when you are drawing out ERD diagrams should you include the "junction" tables as entities, even though they are not explicitly mentioned in the spec, but you can clearly see that it is many to many relationship?
The entities that you include in an ERD really depend upon the intended audience. If you plan on presenting an ERD to software engineers or database administrators then omitting the associative tables would just be confusing. If you are trying to give a high level system overview, then I would advise leaving out everything except the entities that are directly relevant to system operation.
In my personal opinion, junction table aka "cross-walk" tables are important to show data flow.
I think leaving these cross-walk tables out of the ERD, may make viewing the logical flow of the data difficult to comprehend for someone new to the ERD. As your data model becomes more complex, it makes it increasingly difficult to comprehend if you do not show them.
I have a schema with a number of many to many relationships and what I'm seeing is a alot of similar data structure spread out among tables with different names. The intuition I have is that there is a more efficient/desirable way to achieve the same result but I'm not sure what alternative approaches fall into reasonable design/best practices.
Note: Counries, TrafficTypes and People - as they exist now - could all be represented by Id and Name columns, but in the future may have additional fields. Maybe what I'm after is some kind of technique akin to inheritance?
Here's the diagram of what I've got:
Don't lump together things which you think are similar; they may diverge later when you need to store more information about each entity.
Is there a problem with the number of tables you have in your database?
You are probably thinking about the problem from an object oriented design position and thinking you can use some sort of "parent" table to represent the common parts - databases don't work that way.
If you are not careful you will end up with a MUCK or OTLT table.
Off the bat, I would keep seperate enties/objects/(cars vs animals) in seperate tables.
The chances of such enties overlapping properties are slim at best.
Thing is, once those entites start to evolve in your system, you will find that a single table will have hundreds of columns with singles populated per entity.
I don't see how inheritance applies to your case. But if you are interested in inheritance, as it applies to SQL tables or relations, here are some things to look up:
At the level of ER modeling look up "ER model specialization". This is the way the extended ER model diagrams "is A" relationships.
At the level of table design, look up "Class Table Inheritance" and "Shared primary key" for a couple of techniques that, used together, sort of mimic what inheritance does for you in an OOP. You might also want to look up "single table inheritance" for an alternative that's simpler, but can be more wasteful.
Don't worry. You're doing it right.
In a highly normalized schema, you're going to have tons of tables that are nearly identical.
As the others have said, what you're starting to consider (a generic table for multiple things) is a very bad idea and has many drawbacks.
The biggest drawback is that your relationships are made useless by it, in terms of maintaining data integrity. There's nothing stopping someone from assigning a CountryCode where a TrafficTypeId should be, and so on.
Another drawback would be that having one larger table will likely perform worse than many smaller, specialized tables; due to extra, unnecessary blocking.
Your may still want to implement some type of inheritance concept, but that'll be best done in whatever code accesses the database.
I wanna know if there exists conceptual framework or documented techniques to study how to map entities classes to database tables??
Before I was using JPA (Java Persistance API) to map one table -> one entity class, this is: each table's row represented by one object class. Is it the most common/correct way? Has these patterns specific names?
Thanks a lot.
One table to one class is called the "Active Record" pattern. One alternative pattern people use is the Repository pattern. DDD (Domain Driven Design) has the concept of Aggregate Roots which also plays in this space. You should be able to do some reading on that terminology. I am personally not in love with any of those patterns, they all have pros and cons.
For me, there is no recipe, formula or pattern for the ideal class to table relationship. The key point to mapping is that it acts as an adapter layer, isolating the table structure from the entity structure.
If you are designing your application and tables at the same time, I think you should design your tables to minimise round trips, getting as much data to hydrate your entities as possible. Then let your mapper(s) build the entities and any appropriate associations. This might lead you to denormalise your table structure a little more than you might normally.