Define non-prime attribute, What is non-key attribute, Non-key attribute examples, Why they are non-key attributes?
Non-prime (Non-key) attribute
Primary key is an attribute or set of
attributes that are chosen to uniquely identify any records in a table. The
values of a primary key cannot be duplicated. On the other hand, non-prime (non-key)
attributes are the attributes other than the primary key attributes. They can
store a value any many times.
For example, let us consider the
following schemas; (primary keys underlined)
STUDENT (RegNo, Name,
Date_of_Birth, Address)
COURSE_REGISTRATION (RegNo,
Course_No, No_of_Credits, Venue)
In the schema STUDENT, the attribute RegNo
is the primary key (prime attribute). All the other attributes are non-prime
attributes.
In schema COURSE_REGISTRATION, the
attributes RegNo and Course_No together forms the primary key. All the other attributes
are non-prime attributes. In this case, individually RegNo is one prime
attribute and Course_No is another prime attribute.
Why they are called as non-key attributes?
They are non-key attributes because
they cannot uniquely identify any records of the table.
For example, look at the following
queries;
SELECT * FROM
Student WHERE RegNo = ‘12345’;
SELECT * FROM
Student WHERE Name = ‘Kumar’;
Here, RegNo is the primary key of
Student table, so the first query will return zero or one record (if 12345 is a register number). On the other
hand, Name is not a primary key. Hence, the second query will return all the records that stored ‘Kumar’ as
name.
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