Hi Venkatesh,
ER Data model is a conceptual representation of data. It is a database modeling method used to produce a type of conceptual schema or semantic data model of a system. It is very often used in the relational databases. It just defines the roles & relationships between the various participating entities. There are three basic elements in ER models:
1. Entities are the "things" about which we seek information.
2. Attributes are the data we collect about the entities.
3. Relationships provide the structure needed to draw information from multiple entities.
While ER modelling is a database modelling method (you can say a tool), the Extended Star Schema is a type of data warehouse schema. In the star schema design, fact table sits in the middle and is connected to other surrounding objects (dimension lookup tables) like a star. Each dimension is represented as a single table. The primary key in each dimension table is related to a foreign key in the fact table.The star schema is a way to implement multidimensional database (MDDB) functionality using a mainstream relational database.
For example if you want to design a School Attendance system then before you start coding your program you will first need to identify the various entities (the things for which we seek information) and their attributes and then the relationship between the entities. For example in a school hrms system, student can be one entity while class or department can be another. Now the student ID, his date of birth, age etc will become its attributes. Similarly for the entity class, the class id, the class teacher, room number etc can be its attributes. If we talk about the relationships between the entities, then one relationship could be Student "is a member of" Class A. Here the relationship "is a member of" defines how a student is related to class. In this manner we can identify all the entities in the system and create a conceptual representation of their attributes and relationships. Such a diagram is called Entity-Relationship diagram. This ER diagram can later be realized in the form of a data base design where you can have one table for student and one for class. Where the class attribute of the student can be linked to the class id of the class in the form of foreign key relationship.
I hope the example will make the concept if the ER diagram clear to you. The star schema on the on the other hand is an already realized schema, specially designed for multi dimensional databases like infocubes.
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