The "" characteristic of a primary key states that the primary key must uniquely identify each entityinstance, must be able to guarantee unique values, and must not contain nulls.
a. unique values
b. nonintelligent
c. preferably single-attribute
d. security-complaint

A.

Explanation:

Unique means one and only one.

Unique value means primary key cannot have same values. All the values of the primary key are different. Alternatively, every row of primary key has a different value from the other rows of the primary key.

Example: Subject table has the following attributes. snum, sname, description, duration.

These attributes denote subject number, subject name, description of the subject and the duration of the course in hours.

snum is the primary key of the table.

SQL code to create the above table is shows how to declare primary key.

Create table subject

( snum int primary key,

sname varchar2(10),

description varchar2(100),

duration int );

Primary key indicates the following

1.Primary key cannot have null value.

2.Primary key can have only unique value, i.e., primary key cannot have same value.

3.Primary key acts as an index for the table.

Consider the rows of the Subject table which are inserted as shown below.

insert into subject ( snum, sname, description, duration )

values ( 101, 'DBMS', 'fundamentals of database', 40 );

The above row will be executed successfully. This will become the first row in the Subject table.

insert into subject ( snum, sname, description, duration )

values ( 101, ‘OS’, ‘operating system concepts’, 30 )

The above code will not execute since the value of primary key, snum, is the same as the first row. Hence, the code will show an error even if the values for other attributes are different from the first row. Thus, this row will not be inserted.

Alternatively, the below code will execute and another row will be inserted.

insert into subject ( snum, sname, description, duration )

values ( 102, ‘OS’, ‘operating system concepts’, 30 )

Both the rows of Subject table have different values for its primary key, snum.

Due to uniqueness, primary key, snum, acts as the index for the Subject table.

101 indicate DBMS subject and other details. Similarly, 102 denotes OS subject and its details.

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