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It defines that no primary key value can be null. This is as the primary key is used to recognize individual tuple in the relation. So we will not be able to recognize the records uniquely containing null values for the primary key attributes. This constraint is specified on single individual relation.
Example: Let R be the Table
A#
B
C
Null
B1
C1
A2
B2
C2
B3
C3
A4
B4
A5
C5
Note:
1) '#' shows the Primary key of a relation.
In the relation R above, the primary key has null values in the tuples t1 & t3. NULL value in primary key is not allowed, therefore, relation instance is an invalid instance.
Can we delete data element, which is being used by table fields. No we cant delete data element.
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why three level architecture had been developed/
Which product produces the relation that has attributes of R1 and R2 ? Cartesian product produce the relation which has attributes of R1 and R2
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A student entity type has the following attributes,name..
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