Page 252 - Information_Practice_Fliipbook_Class11
P. 252
Answer the following questions:
1. Can a table have two identical tuples?
2. Consider the table MOVIE given below:
+---------+------------------------------+----------+--------+--------------------+
| MOVIEID | TITLE | LANGUAGE | RATING | PLATFORM |
+---------+------------------------------+----------+--------+--------------------+
| M001 | Pushpa: The Rise | Telugu | 7.9 | Amazon Prime Video |
| M004 | The Mauritanian | Spanish | 7.5 | Showtime |
| M010 | Jai Bheem | Tamil | 9.1 | Disney+Hotstar |
| M011 | Spider-Man: No Way Home | Hindi | 8.8 | NA |
| M015 | Thalaivi | French | 6.1 | Netflix |
| M018 | Zack Snyder's Justice League | English | 8.1 | HBO Max |
+---------+------------------------------+----------+--------+--------------------+
What is the degree of the table MOVIE? What is the cardinality of the table MOVIE?
NULL Value
If the value for an attribute is unknown for a tuple, DBMS uses the NULL value. An attribute value may be NULL in the
following situations:
• The value exists but is currently unknown (for example, DOB).
• We do not know whether the value exists (for example, landline number).
• No value is applicable (for example, spouse name for an unmarried employee).
Note that in DEPARTMENT relation, Mgr_Id corresponding to the department having Dept_No 3 is NULL as a
manager has not been assigned to the department having Dept_No 3.
As mentioned above, a key identifies a tuple in a table uniquely. Attributes that are part of the key are known as key
attributes. When a key has more than one attribute, it is called a composite key. For example, in the relation WORKS_
ON, neither Proj_No nor Emp_Id can act as the key of the table WORKS_ON (why?). However, attributes Proj_No
and Emp_Id together form a composite key of the relation WORKS_ON. The notion of keys is described below:
NULL: Denotes an unknown attribute value.
9.2.2 Database keys
1. Candidate Key
A set of attributes that uniquely identify each tuple in a relation is called its candidate key. The set of attributes
that form a candidate key is minimal in the sense that no subset of a candidate key qualifies to be a candidate key.
For example, let us consider a scenario where no two projects have the same project number Proj_No. Further, the
project names are unique within a department. However, different departments may have projects with the same
name. Let us consider candidate keys for the relation PROJECT in this scenario. As no two projects have the same
Proj_No, the attribute Proj_No will serve as a candidate key.
Candidate key: Minimal set of attributes that identifies a tuple.
As Proj_No identifies the tuples in the table PROJECT uniquely, it is evident that the following combinations of the
attributes will also pinpoint the tuples in the table PROJECT uniquely:
Proj_No
Proj_No, Proj_Name
238 Touchpad Informatics Practices-XI

