 Residents in this module I'll go into slightly more detail of what I mentioned in the prior modules which is ER entity relationship modeling. What does ER modeling does? It models, it captures the real world which we are trying to encapsulate in a database on which we will running queries, asking questions, getting answers and that database will be at the back end of your web-enabled database application. It models the real world things which are the entities, okay, so what are in more detail let's go into. So an entity is something which is important in our business or whatever we are modeling if it's a university then the entities are the students, they are the teachers, the course is also, if the entity which we are modeling is a business then employees are the entities, customers are the entities, products are the entities, vendors are the entities. So an entity is represented by what? We are represented by a rectangle. As you can see over here, a student is represented by a rectangle. Now the entities have properties which are the attributes, so those attributes are represented by ellipse. So a student has a name, student has a birth date, student has a roll number, employee has an employee ID, employee has a salary which students usually don't have and employee has an email address and so on. So the attributes are represented by ellipse. Now we can have more details associated with the attributes. For example, if you consider the student name then the student name may have a last name also and may have a first name also in addition to the birth date and the roll number, right or not. And of course there can be entities with attributes such that the attributes are multivalued. A student has a single name. Of course we have the first and the last name but the name is single, however a student may have multiple mobile phones or phone numbers so that is shown by a double line as you can see at the bottom of your screen. This goes into more representative details that you have seen. Now there is one thing more, try to understand that there are certain things which would not be very efficient to store in the database especially those things or those attributes to be more precise which are changing on a daily basis. The GPA does not change on a daily basis. The student GPA changes when the results are declared but the age of an employee is very important because based upon the age the retirement date of the employee is established. Of course the age of a student is also important. Now that age attribute is actually not stored in a database. There is no need to it and we call that as a derived attribute which is shown by the dotted line on your screen. So based upon the birth date which is an attribute stored in the database the age is calculated on a daily basis. Similarly in a library system the ERD for a library there will be how many days have passed since the book was issued and how many days the book has been late and so on. Now between the attributes there are relationships. As I mentioned in the prior modules student entity, course entity so the relationship is the course taken by the student so that is represented by a diamond as you can see on the screen. So entity by a rectangle notation attribute by ellipse multiple attributes by double line derived attribute by a dotted line and a relationship by a diamond. So these are the different relationships four relationships which we have already discussed in the previous module and you can see it you understand it. So we have the different types of relationships and these are the same relationships. They are the same interpretation as we discussed in the previous modules. Now what we have is a representation, a standard representation in the ER model which everybody can understand and should understand. Now we have the participation constraints. Now this relational model is powerful enough to encapsulate the participation constraint. What is a participation constraint? A participation constraint means that every member of the set entity is included in that relationship. Try to understand in this diagram over here every borrower of the loan must every borrower has taken a loan. In a bank their customers which may or may not have gotten a loan right or not so that is shown by a single line but every borrower or every person who has gotten a loan is a member of the set who have taken a loan or say for example in a company every employee cannot be a manager but there are certain employees who are managers so this is the point I am trying to make. And finally generalization and specialization. Specialization is that we include, we lump sum all the things for example a parrot, a crow or a robin they are birds as you can see on the screen. And of course when we go to specialization a person can be an employee can be a customer can be a vendor. They have their attributes also so the notation which you see on the screen that notation is powerful enough to encapsulate to capture these relationships that is the strength of the ERD model. That's all I have for you in this module thank you very much.