Explain the characteristics that make a relation different from a file or table?

Ordering of tuples in a relation

r Relation defined as a set of tuples of elements have no order among them.

Ordering of values within a tuple and an alternative definition of relation

Order of attributes and values is not that important.

As long as correspondence between attributes and values maintained.

Alternative definition of relation

Tuples considered as a set of (<attributes>, <value>) pairs.

Each pairs gives the value of the mapping from an attribute A₁to a value v₁ from dom(A₁).

Use the first definition of relation

Attributes and the values within tuples are ordered.

Values and Nulls in tuple

Represent the values of attributes that may be unknown or may not apply to a tuple.
Meanings for null values

Value Unknown
Value exists but is not available
Attribute does not apply to this tuple (also known as value undefined)

Each value in a tuple is atomic.
Flat relational model

Composite and multi-valued attributes not allowed.
First Normal Form assumption.

Multi-valued attributes

Must be represented by separate relations .

Example: Emp_phone

Composite attributes

Represented only by simple component attributes in basics relational model.

Example- Emp-name

Comments

Define the following terms: Entity, Attribute, Attribute Value, Relationship Instance, Composite Attribute, Multi-valued Attribute, Derived Attribute, Complex Attribute, Key Attribute, Value set(Domain), Recursive Relationships?

Entity – An entity can be a real-world object, either animate or inanimate, that can be easily identifiable. For example, in a school database, students, teachers, classes, and courses offered can be considered as entities. All these entities have some attributes or properties that give them their identity. Attribute – In general, an attribute is a characteristic. In a database management system (DBMS), an attribute refers to a database component, such as a table. It also may refer to a database field. Attributes describe the instances in the row of a database. Attribute Value – Attribute value is the real data of a particular entity for each of its attributes. In other word, associated with each real world entities are certain attributes that describe that entity; value of these attributes for any entity is called attribute value. For example, attribute value of first_name of attribute of student_name can be Gyanendra. Relationship Instance – Each relationship instance ri...

Discuss the difference between the ER model and Relational Model?

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Compare the difference between the following:

i) Entities and Attributes – Entity - An entity can be a real-world object, either animate or inanimate, that can be easily identifiable. For example, in a school database, students, teachers, classes, and courses offered can be considered as entities. All these entities have some attributes or properties that give them their identity. ii) an attribute is a characteristic. In a database management system (DBMS), an attribute refers to a database component, such as a table. It also may refer to a database field. Attributes describe the instances in the row of a database. ii) Composite and Atomic attributes – Composite attribute − Composite attributes are made of more than one simple attribute. For example, a student's complete name may have first_name and last_name. Simple attribute (Atomic) − Simple attributes are atomic values, which cannot be divided further. For example, a student's phone number is an atomic value of 10 digits. iii) Single valued and Mu...

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