dbms keys.pptx
- 1. KEYS: Primary Key: • An attribute or set of attributes that uniquely identifies a row or record in a relation/table is known as primary key. • A relation can have only one Primary Key. • Each value in Primary Key attribute must be unique. • Primary Key cannot contain null values. Candidate Key: • The attributes or set of attributes that can be used as primary key is called Candidate Key. Reg No Name Class 10 Asad C.A 20 Mohsin P.H.D 30 Zeeshan M.B.B.S The attribute Reg No is Primary Key.
- 2. KEYS: Alternate Key: • The candidate keys that are not selected as primary key are know as Alternate Keys. Composite Key: • A primary key that consists of two or more attributes is known as Composite Key.
- 3. KEYS: Secondary Key: • An attribute or set of attributes that is bases for retrieval is called S.K • One Secondary Key value may refer to many records. • Secondary Key can be a non-unique attribute. Foreign Key: • A Foreign Key is an attribute or set of attributes in a relation/table whose values match a primary key in another relation.