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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.
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WHAT IS DATABASE TECHNOLOGIES
Find out the names of all employees who work for first Bank Corporation from database. SELECT EMPLOYEE_NAME FROM WORKS WHERE COMPANYNAME = ‘First Bank Corporation';
For the relations R and S given below: R S A B C 1 4 7 2 5 8 3 6 9
WHAT IS MEANT BY E-R MODEL
Normal 0 false false false EN-US X-NONE X-NONE
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