GATE Computer Science Engineering(CSE) 2027 Test: ER-Model- 1 Free Online

A Data Flow Diagram (DFO) is a graphical representation of the flow of data through as information system, modelling its process aspects.

Relational algebra can not preform the following:
(a) Aggregate computation (avg, sum, etc. must be used).
(b) Multiplication
(c) Finding transitive closure
These operations are beyond the capability of relational algebra.

In an airline reservation system, the primary key should uniquely identify each flight instance. Here, the appropriate primary key would generally be a combination of:

- **Date**

- **Flight number**

This combination uniquely identifies a specific flight on a specific day, as a flight number alone could apply to multiple dates, and a date alone would apply to multiple flights. Other attributes, like place of departure, destination, type of plane, and seats available, can vary but do not uniquely identify a single flight instance.

Thus, the **primary key is the combination of Date and Flight number**.

Every column of the table is referred to as attribute. Row of the table are called as tuples. Number of columns in the table defines the degree of the table.

In a E-R diagram, a rectangle represents entity set, relationship among entity set are represented by diamond box. Attributes are represented by ellipse and the link between attributes and entity set are represented by straight lines.

Domain is defined as the set of permitted values for each attribute. The values of that attribute must lie within the defined domain.

Data models are the basic entities through which abstraction in a DBMS in introduced, It actually defines how data is connected to each other and how they are processed and stored inside the system.

An alternate key is a key associated with one or more columns whose values uniquely identify every row in the table, but where is not the primary key. So, it can be said that an alternate key is a candidate key which is not a primary key.

It is a top-down method.
An entity–relationship model (ER model) describes inter-related things of interest in a specific domain of knowledge. An ER model is composed of entity types (which classify the things of interest) and specifies relationships that can exist between instances of those entity types.

In software engineering an ER model is commonly formed to represent things that a business needs to remember in order to perform business processes. Consequently, the ER model becomes an abstract data model that defines a data or information structure that can be implemented in a database, typically a relational database.

Some ER modelers show super and subtype entities connected by generalization-specialization relationships, and an ER model can be used also in the specification of domain-specific ontologies.

A tuple is a row is a relation. There can exist multiple foreign keys in the same relation because there can exist multiple attributes in the relation that are primary keys in two or more other relations.