What is entity-relationship model? at EXPLAIN EXTENDED
ER model allows you to draw Database Design; It is an easy to use graphical Helps you to define terms related to entity relationship modeling . You need to study the files, forms, reports, data currently maintained by the. An entity-relationship model (ERM) is a theoretical and conceptual way of showing data relationships in software development. ERM is a database modeling. It is well suited to data modelling for use with databases because it is fairly abstract ER models, also called an ER schema, are represented by ER diagrams. .. Use the ERD of a school database in Figure to answer questions 7 to
Learning MySQL by Hugh E. Williams, Saied M.M. Tahaghoghi
A customer has registered A customer has registered A customer has registered …another file to which a new line is added when an order is made: An order is made An order is made An order is made … and a third file to keep the list of items ordered.
An item is ordered An item is ordered … Quite useless, right? However, there still is some information, as per request. We know that each row in the files describe a customer, an order or an item, and we can get the total number of customers, orders and items we've had so far. This is better than nothing but still far from being usable.
Note that the diagram tells us nothing of how they should be stored. We could write them into a file, or on a paper or carve it on a stone. Relationships Diamonds mean relationships.
How the entities are related to each other? We see that we should store the relationships between orders and clients as well as those between items and orders. In our cases, we have a 1: Want to store an order? Be prepared to link it to the piece of information about the client who made it and make sure this information describes exactly one client.
This is the database developer's task. However, a client can have an arbitrary number of orders: The database should provide an ability to store the clients and orders this way. Again, the entity-relationship model does not prescribe how should we store that data, as long as the storage method satisfies the conditions above.
We could store it in two files and relate them using the row numbers: A customer has registered A customer has registered A customer has registered … An order is made by the customer described on line 1 An order is made by the customer described on line 2 An order is made by the customer described on line 2or we can just keep the information in a single text file: The latter one limits us to only 2 clients and 5 orders, but, you know, every system has its limitations.
Attributes Ovals are attributes. What information is stored in the database? Mere enumerating the clients is nice but serves no purpose.
Much better if you know the names of the clients; the orders need to be assigned with unique numbers that help to identify them; and it would be great to record which good did which item contain so not only the number of packages could be checked but their contents too.
This is in fact what the database is for: Not only the links between the entities but the descriptions of the entities too. Entity-relationship and relational model Everything above should be squeezed into relational model, which as we all know stores relations. Since the time relational database appeared, they were mostly used to implement ER models.
Multiple database manuals and guides describe the relational databases solely from that point of view. Various tools exist to automatically generate relational structure given a model.
However, ER model and a relational database are not the same. There is even no mapping to either side: You are attending this lecture I am giving the lecture Just loke entities, we can classify relationships according to relationship-types: A student attends a lecture A lecturer is giving a lecture. Weak Entities A weak entity is a type of entity which doesn't have its key attribute. It can be identified uniquely by considering the primary key of another entity.
Entity Relationship Modeling
For that, weak entity sets need to have participation. Let's learn more about a weak entity by comparing it with a Strong Entity Strong Entity Set Strong entity set always has a primary key.Part 2.3 Types of attributes in ER diagram simple composite single multi valued stored derived
It does not have enough attributes to build a primary key. It is represented by a rectangle symbol. It is represented by a double rectangle symbol. It contains a Primary key represented by the underline symbol. It contains a Partial Key which is represented by a dashed underline symbol. The member of a strong entity set is called as dominant entity set. The member of a weak entity set called as a subordinate entity set.
Primary Key is one of its attributes which helps to identify its member. In a weak entity set, it is a combination of primary key and partial key of the strong entity set. In the ER diagram the relationship between two strong entity set shown by using a diamond symbol. The relationship between one strong and a weak entity set shown by using the double diamond symbol.
The connecting line of the strong entity set with the relationship is single. The line connecting the weak entity set for identifying relationship is double. Attributes It is a single-valued property of either an entity-type or a relationship-type. For example, a lecture might have attributes: An attribute is represented by an Ellipse Types of Attributes Description Simple attribute Simple attributes can't be divided any further.
For example, a student's contact number.