DBMS Part 2
Here is the part 2 of DBMS. If you not yet read part 1, click here to read.
Multi-valued Attributes
Attributes that have more than one value called "Multi -valued attributes ".
We can graphically represent Multi-valued attributes in ER-Diagram like following,
Derived Attributes
Sometimes value of one attribute can get using another attribute (Some attributes can derived from another attribute).
Ex - You can make "Age" attribute using "Current Date" and "Date of Birth".
Key Attributes
A Key Attribute is unique characteristic of entity set.
Candidate Key and Primary Key
In your database there can be more than one Candidate key. A candidate key is a attribute that use to identify data uniquely. Each entity has more than one "Candidate key".
Among those candidate keys we will choose most suitable key to identify data uniquely. This special key called as a "PRIMARY KEY".
In database there are only one PRIMARY KEY.
Composite Key
When the primary key is created using 2 or more attributes/columns, this kind of primary key called composite key.
Super Key
A super key is a set of one or more attributes that are taken collectively. Also super key can identify all other attributes uniquely.
Ex - We are having table,
BOOK(BookID , BookAutor, BookName)
Here is the possible super keys, each super key is able to uniquely identify each tuple(record).
- BookID
- (BookID , BookName)
- (BookID , Author)
Article By - Nisal Priyanka
