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4. The order in which names of the attributes appear in a relation schema is not relevant to the DBMS for its
operations. Similarly, DBMS ignores the ordering of tuples in a relation.
Can a table have two identical tuples?
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 Disnep+
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?
4.2.2 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.
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. 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
Database Query using SQL 129

