201 - Databases Flashcards
What does data abstraction do in a database systems?
It presents a logical view of data to applications
There are two views of data within the DBMS….
- Logical view, presented to applications
2. Physical layout on backing store as manipulated by the DBMS
What is data independence?
The ability to organise and reorganise the contents and the structure of the data
What is a row and column header called in the relational model?
Tulles and Attributes
What is the data type that describes the type of values that appear in each column?
Domain type
What does Atomic mean?
Each value in domain is indivisible
Data types for domains
- Characters
- Strings
- Date, Time, Timestamp
What is a relation schema?
It’s used to describe a relation
What is a degree?
The degree of relation is the number of attributes n of its relation schema
Are tuples in a relation distinct or not?
They are distinct
What is a super key?
A subset of attributes where no two tuples in any relation state r of R can have the same combination of values for these attributes
What is a candidate key?
When in a relation schema, it has more than one key
What is a primary key?
The candidate key whose values are used to identify tuples in a relation
(Cannot be null)
What is a foreign key?
When a tiple in one relation refers to a tuple in another relation
Why do we need keys?
- They are used to identify tulles in a relation R
- They are essential elements of any relational database
- They are used to establish the relationships amount tables in a schema
