Asimple primary keyconsists ofonly one columnthat uniquely identifies each row in a table.
Example Usage:
sql
CREATE TABLE Students (
StudentID INT PRIMARY KEY,
Name VARCHAR(50)
);
* StudentIDis asimple primary keybecause it consists of only one field.
Why Other Options Are Incorrect:
* Option B (Partition) (Incorrect):Refers topartitioned tables, which divide data for performance reasons butare not related to primary keys.
* Option C (Stable) (Incorrect):This is not a recognized term in database keys.
* Option D (Meaningless) (Incorrect):Primary keys are oftenmeaningless (e.g., auto-incremented IDs)
, but this isnot a term used to describe their structure.
Thus, the correct answer isSimple, as asingle-field primary keyis referred to as asimple primary key.

TheCASCADEconstraint ensures thatupdates or deletions in the primary key table automatically reflect in the foreign key table.
Example Usage:
sql
CREATE TABLE Departments (
DeptID INT PRIMARY KEY,
DeptName VARCHAR(50)
);
CREATE TABLE Employees (
EmpID INT PRIMARY KEY,
Name VARCHAR(50),
DeptID INT,
FOREIGN KEY (DeptID) REFERENCES Departments(DeptID) ON UPDATE CASCADE ON DELETE CASCADE );
* If DeptIDchangesin Departments, itautomatically updatesin Employees.
* If a DeptID isdeleted, all employees in that departmentare also deleted.
Why Other Options Are Incorrect:
* Option A (SET DEFAULT) (Incorrect):Sets foreign key values to adefaultvalue, rather than propagating changes.
* Option B (SET NULL) (Incorrect):When the referenced key is deleted, dependent records areset to NULLinstead of being updated/deleted.
* Option C (RESTRICT) (Incorrect):Prevents deletion of a referenced row if dependent foreign key rows exist.
Thus, the correct answer isCASCADE, as itpropagates primary key changes to dependent foreign keys.

The given diagram represents a unary relationship (also called a recursive relationship), which occurs when an entity is related to itself. In this case, salespersons back each other up, meaning a salesperson is associated with another salesperson from the same entity.
Key Observations from the Diagram:
* Single Entity (Salesperson)
* The table contains only one entity type, Salesperson, which has attributes such as Salesperson Number, Name, Commission, Percentage, and Year of Hire.
* Self-Referencing Relationship (Backs-up and Backed-up by)
* The diagram shows that a salesperson can back up another salesperson.
* This means that the relationship exists within the same entity rather than between two different entities.
* Cardinality (One-to-One Relationship in a Unary Form)
* The notation ( |-| ) in the ER diagram indicates a one-to-one recursive relationship.
* One salesperson can back up only one other salesperson, and each salesperson is backed up by only one.

Attributesin a databaserepresent the properties (names, values, or characteristics) of an entity.
Example Usage:
* In aStudentstable, attributes might include:
StudentID (Primary Key), Name, Age, Major
* Here,Name is an attributedescribing the entityStudent.
Why Other Options Are Incorrect:
* Option A (Synonyms) (Incorrect):Synonyms in SQL allow different names for the same object but are not attributes.
* Option C (Repositories) (Incorrect):A repository stores data butdoes not define attributes.
* Option D (Relationships) (Incorrect):Relationships defineconnections between entities, not their attributes.
Thus, the correct answer isNames, asattributes define entity properties.

Aunary relationship(also known as arecursive relationship) occurs when an entityrelates to itself.
Example Usage:
* Employees and Managers:
sql
CREATE TABLE Employees (
EmpID INT PRIMARY KEY,
Name VARCHAR(50),
ManagerID INT,
FOREIGN KEY (ManagerID) REFERENCES Employees(EmpID)
);
* Here, ManagerIDreferences another Employee# aunary (self-referential) relationship.
Why Other Options Are Incorrect:
* Option A (Modality) (Incorrect):Describesoptional vs. mandatoryrelationships, not self-referencing.
* Option C (Cardinality) (Incorrect):Defines how many instances relate,not the type of relationship.
* Option D (Binary) (Incorrect):Binary relationships involvetwo different entities, not self-referencing.
Thus, the correct answer isUnary, as it describesrelationships within the same entity type.