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प्रश्न
Why foreign keys are allowed to have NULL values? Explain with an example.
उत्तर
The value of the foreign key is derived from the Primary key of the primary table. Sometimes, we want to enter a record, which is not related. To handle this situation, it is allowed to enter a NULL value in the foreign key.
For example, Suppose a shopkeeper wants to sell a product, but the customer is not a regular customer, so his customer id does not exist. In this case, a NULL value is entered into the customerId field.
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संबंधित प्रश्न
Minimal superkeys are known as?
The attributes which have all the properties of primary key
The primary key can be?
______ refers to the attribute that can uniquely identify tuples within the relation.
An Alternate key can be defined as ______.
An attribute in a table whose value is derived from primary key of another table is known as ______.
Which of the following is true for a tuple and attribute?
Differentiate between Primary key and foreign key.
A school has a rule that each student must participate in a sports activity. So each one should give only one preference for sports activity. Suppose there are five students in a class, each having a unique roll number. The class representative has prepared a list of sports preferences as shown below. Answer the following:
| Sports Preferences | |
| Roll_no | Preference |
| 9 | Cricket |
| 13 | Football |
| 17 | Badminton |
| 17 | Football |
| 21 | Hockey |
| 24 | NULL |
| NULL | Kabaddi |
- Roll no 24 may not be interested in sports. Can a NULL value be assigned to that student’s preference field?
- Roll no 17 has given two preference sports. Which property of relational DBMC is violated here? Can we use any constraint or key in the relational DBMS to check against such violations, if any?
- Kabaddi was not chosen by any student. Is it possible to have this tuple in the Sports Preferences relation?
The school canteen wants to maintain records of items available in the school canteen and generate bills when students purchase any item from the canteen. The school wants to create a canteen database to keep track of items in the canteen and the items purchased by students. Design a database by answering the following question:
In order to generate the bill, we should know the quantity of an item purchased. Should this information be in a new relation or a part of the previous relation? If a new relationship is required, decide the appropriate name and data type for attributes. Also, identify the appropriate primary key and foreign key so that the following two restrictions are satisfied:
- The same bill cannot be generated for different orders.
- Bill can be generated only for available items in the canteen.
An organisation wants to create a database EMPDEPENDENT to maintain the following details about its employees and their dependent.
EMPLOYEE(AadharNumber, Name, Address, Department, EmployeeID)
DEPENDENT(EmployeeID, DependentName, Relationship)
- Name the attributes of EMPLOYEE, which can be used as candidate keys.
- The company wants to retrieve details of the dependent of a particular employee. Name the tables and the key which are required to retrieve this detail.
- What is the degree of EMPLOYEE and DEPENDENT relation?
In a multiplex, movies are screened in different auditoriums. One movie can be shown in more than one auditorium. In order to maintain the record of movies, the multiplex maintains a relational database consisting of two relations viz. MOVIE and AUDI respectively as shown below:
Movie(Movie_ID, MovieName, ReleaseDate) Audi(AudiNo, Movie_ID, Seats, ScreenType, TicketPrice)
Is it correct to assign Movie_ID as the primary key in the MOVIE relation? If no, then suggest an appropriate primary key.
In a multiplex, movies are screened in different auditoriums. One movie can be shown in more than one auditorium. In order to maintain the record of movies, the multiplex maintains a relational database consisting of two relations viz. MOVIE and AUDI respectively as shown below:
Movie(Movie_ID, MovieName, ReleaseDate) Audi(AudiNo, Movie_ID, Seats, ScreenType, TicketPrice)
- Is it correct to assign Movie_ID as the primary key in the MOVIE relation? If no, then suggest an appropriate primary key.
- Is it correct to assign AudiNo as the primary key in the AUDI relation? If no, then suggest an appropriate primary key.
- Is there any foreign key in any of these relations?
| Student Project Database | ||||
| Table: STUDENT | ||||
| Roll No | Name | Class | Section | Registration_ID |
| 11 | Mohan | XI | 1 | IP-101-15 |
| 12 | Sohan | XI | 2 | IP-104-15 |
| 21 | John | XII | 1 | CS-103-14 |
| 22 | Meena | XII | 2 | CS-101-14 |
| 23 | Juhi | XII | 2 | CS-101-10 |
| Table: PROJECT | ||||
| ProjectNo | PName | SubmissionDate | ||
| 101 | Airline Database | 12/01/2018 | ||
| 102 | Library Database | 12/01/2018 | ||
| 103 | Employee Database | 15/01/2018 | ||
| 104 | Student Database | 12/01/2018 | ||
| 105 | Inventory Database | 15/01/2018 | ||
| 106 | Railway Database | 15/01/2018 | ||
| PROJECT ASSIGNED | ||||
| Registration_ID | ProjectNo | |||
| IP-101-15 | 101 | |||
| IP-104-15 | 103 | |||
| CS-103-14 | 102 | |||
| CS-101-14 | 105 | |||
| CS-101-10 | 104 | |||
- Name the primary key of each table.
- Find foreign key(s) in table PROJECT-ASSIGNED.
- Is there any alternate key in table STUDENT? Give justification for your answer.
- Can a user assign duplicate value to the field RollNo of STUDENT table? Justify.
| Student Project Database | ||||
| Table: STUDENT | ||||
| Roll No | Name | Class | Section | Registration_ID |
| 11 | Mohan | XI | 1 | IP-101-15 |
| 12 | Sohan | XI | 2 | IP-104-15 |
| 21 | John | XII | 1 | CS-103-14 |
| 22 | Meena | XII | 2 | CS-101-14 |
| 23 | Juhi | XII | 2 | CS-101-10 |
| Table: PROJECT | ||||
| ProjectNo | PName | SubmissionDate | ||
| 101 | Airline Database | 12/01/2018 | ||
| 102 | Library Database | 12/01/2018 | ||
| 103 | Employee Database | 15/01/2018 | ||
| 104 | Student Database | 12/01/2018 | ||
| 105 | Inventory Database | 15/01/2018 | ||
| 106 | Railway Database | 15/01/2018 | ||
| PROJECT ASSIGNED | ||||
| Registration_ID | ProjectNo | |||
| IP-101-15 | 101 | |||
| IP-104-15 | 103 | |||
| CS-103-14 | 102 | |||
| CS-101-14 | 105 | |||
| CS-101-10 | 104 | |||
For the above-given database STUDENT-PROJECT, can we perform the following operations?
- Insert a student record with a missing roll number value.
- Insert a student record with a missing registration number value.
- Insert a project detail without a submission date.
- Insert a record with registration ID IP-101-19 and ProjectNo 206 in the table PROJECT-ASSIGNED.
| Student Project Database | ||||
| Table: STUDENT | ||||
| Roll No | Name | Class | Section | Registration_ID |
| 11 | Mohan | XI | 1 | IP-101-15 |
| 12 | Sohan | XI | 2 | IP-104-15 |
| 21 | John | XII | 1 | CS-103-14 |
| 22 | Meena | XII | 2 | CS-101-14 |
| 23 | Juhi | XII | 2 | CS-101-10 |
| Table: PROJECT | ||||
| ProjectNo | PName | SubmissionDate | ||
| 101 | Airline Database | 12/01/2018 | ||
| 102 | Library Database | 12/01/2018 | ||
| 103 | Employee Database | 15/01/2018 | ||
| 104 | Student Database | 12/01/2018 | ||
| 105 | Inventory Database | 15/01/2018 | ||
| 106 | Railway Database | 15/01/2018 | ||
| PROJECT ASSIGNED | ||||
| Registration_ID | ProjectNo | |||
| IP-101-15 | 101 | |||
| IP-104-15 | 103 | |||
| CS-103-14 | 102 | |||
| CS-101-14 | 105 | |||
| CS-101-10 | 104 | |||
For the above-given database STUDENT-PROJECT, can we perform the following operation?
Insert a student record with a missing registration number value.
Explain the use of ‘Foreign Key’ in a Relational Database Management System. Give example to support your answer.
Explain the concept of “Alternate Key” in a Relational Database Management System with an appropriate example.
Which of the following statements is FALSE about keys in a relational database?
Give one difference between alternate key and candidate key.
