1. Composite Attribute
An attribute composed of many other attributes is called a composite attribute. For example,
Address attribute of student, Entity type consists of Street, City, State, and Country.
2. Derived Attribute
A derived attribute is an attribute whose values are calculated from other attributes. In a student
table if we have an attribute called as date_of_birth and age.
3. Strong Entity
A strong entity is an entity type whose existence doesn't depend on any other entity. Strong
entity types have a key attribute. The key attribute helps in identifying each entity uniquely. It is
represented by a rectangle.
4. Weak Entity
Weak Entity does not have a primary key. Weak entity is dependent. It must use a foreign key in
conjunction with its attributes to create a primary key.
5. Purpose of DBMS
Database Management Systems (DBMS) are software systems used to store, retrieve, and run
queries on data. A DBMS serves as an interface between an end-user and a database, allowing
users to create, read, update, and delete data in the database.
6. Natural join
A NATURAL JOIN is a JOIN operation that creates an implicit join clause for you based on the
common columns in the two tables being joined. Common columns are columns that have the
same name in both tables. A NATURAL JOIN can be an INNER join, a LEFT OUTER join, or a
RIGHT OUTER join. The default is INNER join.
7. Intersection vs from natural joins
The INNER JOIN will return duplicates, if id is duplicated in either table. INTERSECT removes
duplicates. The INNER JOIN will never return NULL , but INTERSECT will return NULL.
9. Domain constarint
Domain Constraints are user-defined columns that help the user to enter the value according to
the data type. Examples of domain constraints: Valid value sets. These are valid translation
values for a particular data item. These include code tables, translation tables, and existence
check tables.
10. Domain constarint vs Key Constraint
Domain Constraints are user-defined columns that help the user to enter the value according to
the data type. Examples of domain constraints: Valid value sets. These are valid translation
values for a particular data item. These include code tables, translation tables, and existence
check tables.
Keys are the entity set that is used to identify an entity within its entity set uniquely. An entity set
can have multiple keys, but out of which one key will be the primary key. A primary key can
contain a unique and null value in the relational table.
11 . Generalization
Generalization is the process of extracting common properties from a set of entities and create
a generalized entity from it. It is a bottom-up approach in which two or more entities can be
generalized to a higher level entity if they have some attributes in common. For Example,
STUDENT and FACULTY can be generalized to a higher level entity called PERSON as shown
in Figure 1. In this case, common attributes like P_NAME, P_ADD become part of higher entity
(PERSON) and specialized attributes like S_FEE become part of specialized entity (STUDENT).
Specialization
In specialization, an entity is divided into sub-entities based on their characteristics. It is a
top-down approach where higher level entity is specialized into two or more lower level entities.
For Example, EMPLOYEE entity in an Employee management system can be specialized into
DEVELOPER, TESTER etc. as shown in Figure 2. In this case, common attributes like
E_NAME, E_SAL etc. become part of higher entity (EMPLOYEE) and specialized attributes like
TES_TYPE become part of a specialized entity (TESTER).
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. Aggregation is an abstraction through which we
can represent relationships as higher level entity sets.
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.