keys

Types of Keys:

• Primary Key (PK): Unique tuple ident; no nulls, no duplicates.
• Foreign Key (FK): Links two tables; refs PK in parent tbl, ensures referential integrity.
• Candidate Key: All potential PKs; unique, irreducible.
• Alternate Key: Candidate keys not chosen as PK.
• Composite Key: Combines multiple attrs to form a unique id.
• Super Key: Attr set uniquely idents tuples; may contain extra attrs.

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Primary Key:

• Props: Ensures uniqueness, non-null values, supports indexing.
• Example: StudentID in Student tbl uniquely identifies each student.

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Foreign Key:

• Props: Refs PK of parent tbl, enforces relationships, can allow nulls/duplicates (optional).
• Example: CourseID in Enrollment tbl refs CourseID in Courses tbl to maintain valid relationships.

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Simple v/s Composite Keys`

Aspect	Simple Key	Composite Key
Definition	A single attribute uniquely identifies tuples.	Combines multiple attributes to uniquely identify tuples.
Complexity	Simpler to define, manage, and index.	More complex due to multiple attributes.
Example	EmployeeID in an Employee table.	{OrderID, ProductID} in an OrderDetails table.
Key Size	Single column/field.	Multiple columns/fields.
Indexing	Efficient and straightforward.	Slower indexing due to combined attributes.
Query Performance	Faster as only one attribute is involved.	Slower for large datasets due to joins on multiple attributes.
Uniqueness	Ensures tuple uniqueness with one attribute.	Ensures tuple uniqueness through a combination of attributes.
Usability in FK	Easier to reference as FK in another table.	More difficult to reference as FK; requires all attributes in combination.
Use Case	When a single attribute is sufficient for identification.	When no single attribute can uniquely identify tuples.
Storage Requirement	Requires less storage.	Requires more storage due to multiple attributes.