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Differentiate between various levels of data abstraction?
Data Abstraction - Abstraction is the procedure to hide the irrelevant things from the users and represent the relevant things to the user. Database systems are frequent used through non-computer professionals so in which the complexity must be hidden from database system users. This is done through defining levels of abstract as that the database may be viewed; there is logical view or external view, conceptual view and internal view or physical view.
1. External View - This is the highest level of abstraction as seen through a user. It describes only the part of whole database, that is relevant to a particular user.
2. Conceptual View - This is the next higher stages of abstraction that is the sum total of Database Management System user's views. It elaborates what data are in fact stored in the database. It contains information about whole database in terms of a small number of associatively easy structure.
3. Internal View - This is the lowest level of abstraction. It elaborates how the data are physically stored
The need for three level architecture The aim of the three level architecture is to divide each user's view of the database from the way the database is physically shown.
Views in relational algebra: 1. Basic expression contains either a) A relation in the database. b) A constant relation. 2. General expressions are madde out of sma
Describe an data dictionary? A data dictionary is a data structure which keeps meta data about the structure of the database ie. the schema of the database.
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