DBMS Generalization Generalization is a process in which the common attributes of more than one entities form a new entity. This newly formed entity is called generalized entity. Generalization Example Lets say we have two entities Student and Teacher. Attributes of Entity Student are: Name, Address & Grade Attributes of Entity Teacher are: Name, Address & Salary The ER diagram before generalization looks like this: These two entities have two common attributes: Name and Address, we can make a generalized entity with these common attributes. Lets have a look at the ER model after generalization. The ER diagram after generalization: We have created a new generalized entity Person and this entity has the common attributes of both the entities. As you can see in the following ER diagram that after the generalization process the entities Student and Teacher only has the specialized attributes Grade and Salary respectively and their common attributes (Name & Address) are now associated with a new entity Person which is in the relationship with both the entities (Student & Teacher). Note: 1. Generalization uses bottom-up approach where two or more lower level entities combine together to form a higher level new entity. 2. The new generalized entity can further combine together with lower level entity to create a further higher level generalized entity. Generalization As mentioned above, the process of generalizing entities, where the generalized entities contain the properties of all the generalized entities, is called generalization. In generalization, a number of entities are brought together into one generalized entity based on their similar characteristics. For example, pigeon, house sparrow, crow and dove can all be generalized as Birds. Generalization is a bottom-up approach in which two lower level entities combine to form a higher level entity. In generalization, the higher level entity can also combine with other lower level entities to make further higher level entity. It's more like Superclass and Subclass system, but the only difference is the approach, which is bottom-up. Hence, entities are combined to form a more generalised entity, in other words, sub-classes are combined to form a superclass. For example, Saving and Current account types entities can be generalised and an entity with name Account can be created, which covers both. Specialization Specialization is the opposite of generalization. In specialization, a group of entities is divided into sub-groups based on their characteristics. Take a group ‘Person’ for example. A person has name, date of birth, gender, etc. These properties are common in all persons, human beings. But in a company, persons can be identified as employee, employer, customer, or vendor, based on what role they play in the company. Similarly, in a school database, persons can be specialized as teacher, student, or a staff, based on what role they play in school as entities. Specialization Specialization is opposite to Generalization. It is a top-down approach in which one higher level entity can be broken down into two lower level entity. In specialization, a higher level entity may not have any lower-level entity sets, it's possible. Aggregration Aggregration is a process when relation between two entities is treated as a single entity. In the diagram above, the relationship between Center and Course together, is acting as an Entity, which is in relationship with another entity Visitor. Now in real world, if a Visitor or a Student visits a Coaching Center, he/she will never enquire about the center only or just about the course, rather he/she will ask enquire about both. Aggregation – An ER diagram is not capable of representing relationship between an entity and a relationship which may be required in some scenarios. In those cases, a relationship with its corresponding entities is aggregated into a higher level entity. For Example, Employee working for a project may require some machinery. So, REQUIRE relationship is needed between relationship WORKS_FOR and entity MACHINERY. Using aggregation, WORKS_FOR relationship with its entities EMPLOYEE and PROJECT is aggregated into single entity and relationship REQUIRE is created between aggregated entity and MACHINERY. What is the Super key? A superkey is a group of single or multiple keys which identifies rows in a table. A Super key may have additional attributes that are not needed for unique identification. Example: EmpSSN EmpNum Empname 9812345098 AB05 Shown 9876512345 AB06 Roslyn 199937890 AB07 James In the above-given example, EmpSSN and EmpNum name are superkeys. What is a Primary Key? A column or group of columns in a table which helps us to uniquely identifies every row in that table is called a primary key. This DBMS can't be a duplicate. The same value can't appear more than once in the table. Rules for defining Primary key: • • • • Two rows can't have the same primary key value It must for every row to have a primary key value. The primary key field cannot be null. The value in a primary key column can never be modified or updated if any foreign key refers to that primary key. Example: In the following example, <code>StudID</code> is a Primary Key. StudID Roll No First Name LastName Email 1 11 Tom Price abc@gmail.com 2 12 Nick Wright xyz@gmail.com 3 13 Dana Natan mno@yahoo.com What is the Alternate key? All the keys which are not primary key are called an alternate key. It is a candidate key which is currently not the primary key. However, A table may have single or multiple choices for the primary key. Example: In this table. StudID, Roll No, Email are qualified to become a primary key. But since StudID is the primary key, Roll No, Email becomes the alternative key. StudID Roll No First Name LastName Email 1 11 Tom Price abc@gmail.com 2 12 Nick Wright xyz@gmail.com 3 13 Dana Natan mno@yahoo.com What is a Candidate Key? A super key with no repeated attribute is called candidate key. The Primary key should be selected from the candidate keys. Every table must have at least a single candidate key. Properties of Candidate key: • • • • • It must contain unique values Candidate key may have multiple attributes Must not contain null values It should contain minimum fields to ensure uniqueness Uniquely identify each record in a table Example: In the given table Stud ID, Roll No, and email are candidate keys which help us to uniquely identify the student record in the table. StudID Roll No First Name LastName Email 1 11 Tom Price abc@gmail.com 2 12 Nick Wright xyz@gmail.com 3 13 Dana Natan mno@yahoo.com What is the Foreign key? A foreign key is a column which is added to create a relationship with another table. Foreign keys help us to maintain data integrity and also allows navigation between two different instances of an entity. Every relationship in the model needs to be supported by a foreign key. What is the Compound key? Compound key has many fields which allow you to uniquely recognize a specific record. It is possible that each column may be not unique by itself within the database. However, when combined with the other column or columns the combination of composite keys become unique. Example: OrderNo PorductID Product Name Quantity B005 JAP102459 Mouse 5 B005 DKT321573 USB 10 B005 OMG446789 LCD Monitor 20 B004 DKT321573 USB 15 B002 OMG446789 Laser Printer 3 In this example, OrderNo and ProductID can't be a primary key as it does not uniquely identify a record. However, a compound key of Order ID and Product ID could be used as it uniquely identified each record. What is the Composite key? A key which has multiple attributes to uniquely identify rows in a table is called a composite key. The difference between compound and the composite key is that any part of the compound key can be a foreign key, but the composite key may or maybe not a part of the foreign key Definition of Composite key: A key that has more than one attributes is known as composite key. It is also known as compound key. Note: Any key such as super key, primary key, candidate key etc. can be called composite key if it has more than one attributes.
