when foreign  key constraint is created on a column 
referential integrity is achieved .
when foreign key constraint is created on a column,that 
column reference other column in the different table or 
even in the same table but the reference column must be 
primary key or unique key.
when a new value is added in a column referenced with 
foreign key ,then the foreign key relation checks whether 
the new value matches with the reference column or not.
If value matches then they  are allowed into the table

Cursors can be either updateable or non-updateable. If you 
only need to display information and not diddle with it, 
the non-updateable is the fastest. The provider simply 
passes you the data and forgets about it! There is no need 
to keep tabs on the data to see if it has been changed. 
Therefore, this is the fastest cursor to use.



Cursor type

Static cursor. This is the one to use for generating 
reports or finding data. Additions, changes, or deletions 
by other users are not visible

Forward-only cursor. This is the default. It is identical 
to the Static except that you can only scroll forward. The 
fastest cursor this side of the Pecos Mountains.

Dynamic cursor. Additions and deletions by others are 
visible. All movement is supported. But some providers 
don't support this cursor type.

Keyset-driven cursor. This is similar to a Dynamic cursor 
except you can't see records others add. If another user 
deletes a record, it is inaccessible from your recordset