Relational Questions and Answers
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1 :: Explain DML (Data Manipulation Language)?

This language that enable user to access or manipulate data as organised by appropriate data model.

► Procedural DML or Low level: DML requires a user to specify what data are needed and how to get those data.
► Non-Procedural DML or High level: DML requires a user to specify what data are needed without specifying how to get those data.
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2 :: Explain VDL (View Definition Language)?

It specifies user views and their mappings to the conceptual schema.
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3 :: What you know in mapping of ERD to DFD?

a) entities in ERD should correspond to an existing entity/store in DFD
b) entity in DFD is converted to attributes of an entity in ERD
c) relations in ERD has 1 to 1 correspondence to processes in DFD
d) relationships in ERD has 1 to 1 correspondence to flows in DFD

(a) entities in ERD should correspond to an existing entity/store in DFD
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4 :: Explain Data Independence?

Data independence means that ?the application is independent of the storage structure and access strategy of data?. In other words, The ability to modify the schema definition in one level should not affect the schema definition in the next higher level.

Two types of Data Independence are:

► Physical Data Independence: Modification in physical level should not affect the logical level.
► Logical Data Independence: Modification in logical level should affect the view level.
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5 :: Explain Armstrong rules? How they are complete and/or sound?

The well-known inference rules for FDs
►► Reflexive rule :
If Y is subset or equal to X then X Y.
►► Augmentation rule:
If X Y then XZ YZ.
►► Transitive rule:
If {X Y, Y Z} then X Z.
►► Decomposition rule :
If X YZ then X Y.
►► Union or Additive rule:
If {X Y, X Z} then X YZ.
►► Pseudo Transitive rule :
If {X Y, WY Z} then WX Z.

Of these the first three are known as Amstrong Rules. They are sound because it is enough if a set of FDs satisfy these three. They are called complete because using these three rules we can generate the rest all inference rules.
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