Explain the following terms: 1. Data Dictionary 2. Weak Entity

Explain the following terms: 1. Data Dictionary 2. Weak Entity

Answer:-

1. Data Dictionary

A data dictionary (also known as a metadata repository) is like a “catalog” for your database. It stores definitions and information about all database objects. Here’s what it typically includes:

• Table information: Names of tables and their columns
• Column details: Data types, default values, constraints (e.g., NOT NULL)
• Relationships: Which keys link tables together (primary vs. foreign keys)
• Indexes: Names and types of indexes used
• Privileges: Who can access or modify data

Why it matters:

• Helps DBAs and developers understand database structure
• Ensures consistency in naming and types
• Aids in maintenance, documentation, and impact assessment during changes

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2. Weak Entity

A weak entity is an entity that cannot be uniquely identified by its own attributes alone. It depends on another entity (called its owner or parent) to ensure uniqueness.

Key Characteristics:

• Has a partial key (an attribute that identifies instances only within the context of its owner)
• Participates in an identifying relationship with its owner entity
• Often requires a composite primary key combining its partial key and the owner’s primary key

Example:

• Owner Entity: Department with primary key DeptID
• Weak Entity: Dependent of an employee
  • Partial key: DependentName (e.g., “Alice”, “Bob”)
  • To uniquely identify, you need (EmployeeID, DependentName)
  • This weak entity is linked to an employee by the identifying relationship “has dependent”

Term	Definition
Data Dictionary	A metadata repository containing table, column, key, index, and user privilege information
Weak Entity	An entity that needs its owner’s key plus its own partial key to form a unique primary key

3. Cardinality Ratio in DBMS

Cardinality Ratio defines the number of entities that can be associated with another entity in a relationship. It tells us how many instances of one entity relate to how many instances of another.

It is an important concept in Entity-Relationship (E-R) modeling.

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Types of Cardinality Ratios:

1. One-to-One (1:1)
   • Definition: One entity in set A is related to at most one entity in set B, and vice versa.
   • Example:
     Each person has one passport, and each passport is assigned to only one person.
2. One-to-Many (1:N)
   • Definition: One entity in set A can be related to many entities in set B, but each entity in B is related to only one in A.
   • Example:
     One department has many employees, but each employee belongs to one department.
3. Many-to-One (N:1)
   • Definition: Many entities in set A can be related to one entity in set B.
   • Example:
     Many students are guided by one professor.
4. Many-to-Many (M:N)
   • Definition: Many entities in A can be related to many entities in B.
   • Example:
     Students enroll in multiple courses, and each course has many students.

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Summary Table:

Type	Notation	Example
One-to-One	1:1	Person ↔ Passport
One-to-Many	1:N	Department → Employees
Many-to-One	N:1	Students → Professor
Many-to-Many	M:N	Students ↔ Courses