Wednesday, 20 March 2024
20 मार्च की महत्त्वपूर्ण घटनाएँ
Monday, 11 March 2024
कर्म और धर्म
Thursday, 7 March 2024
7 मार्च की महत्त्वपूर्ण घटनाएँ
Thursday, 29 February 2024
Sanchar Saathi Web Portal Kya hai
Tarang Sanchar Web Portal Kya hai
Tuesday, 20 February 2024
Projection and Selection Practice Questions in relational algebra
Projection and Selection Practice Questions:
Student
S_ID |
NAME |
Branch |
1 |
A |
CS |
2 |
B |
IT |
3 |
C |
CS |
4 |
D |
CS |
5 |
E |
EE |
6 |
F |
EC |
7 |
G |
EE |
- Find the names of all students from CS branch.
Π name (σ Branch = ‘CS’ (Student))
Account (account_no, branch_name, balance)
- Find those account numbers where balance is less than 1000.
Π account_no (σ balance<1000 (Account))
Loan(loan_no, branch_name, amount)
- Find those loan numbers which are from Delhi branch with amount greater than 1000.
Π loan_no (σ branch_name=’Delhi’ ^ amount>1000 (Loan))
Branch (Branch_name, Branch_city, assets)
- Find branch_name and branch_city with assets more than 100000.
Π branch_name , branch_city (σ assets>100000 (Branch))
Projection Operator in Relational Algebra
Projection Operator-
- Projection Operator (π) is a unary operator in relational algebra that performs a projection operation.
- It displays the columns of a relation or table based on the specified attributes.
- It is a fundamental / Basic operator
- It is a unary operator
Syntax-
π<attribute list>(R) |
Example-
Consider the following Student relation-
ID | Name | Subject | Age |
100 | Ashish | Maths | 19 |
200 | Rahul | Science | 20 |
300 | Naina | Physics | 20 |
400 | Sameer | Chemistry | 21 |
Student
Result for Query πName, Age(Student)-
Name | Age |
Ashish | 19 |
Rahul | 20 |
Naina | 20 |
Sameer | 21 |
Result for Query πID , Name(Student)-
ID | Name |
100 | Ashish |
200 | Rahul |
300 | Naina |
400 | Sameer |
Important Points-
Point-01:
- The degree of output relation (number of columns present) is equal to the number of attributes mentioned in the attribute list.
Point-02:
- Projection operator automatically removes all the duplicates while projecting the output relation.
- So, cardinality of the original relation and output relation may or may not be same.
- If there are no duplicates in the original relation, then the cardinality will remain same otherwise it will surely reduce.
Point-03:
- If attribute list is a super key on relation R, then we will always get the same number of tuples in the output relation.
- This is because then there will be no duplicates to filter.
Point-04:
- Projection operator does not obey commutative property i.e.
π <list2> (π <list1> (R)) ≠ π <list1> (π <list2> (R))
Point-05:
- Selection Operator performs horizontal partitioning of the relation.
- Projection operator performs vertical partitioning of the relation.
Point-06:
- There is only one difference between projection operator of relational algebra and SELECT operation of SQL.
- Projection operator does not allow duplicates while SELECT operation allows duplicates.
- To avoid duplicates in SQL, we use “distinct” keyword and write SELECT distinct.
- Thus, projection operator of relational algebra is equivalent to SELECT operation of SQL.
Selection Operator in Relational Algebra
Selection
Operator-
·
Selection Operator (σ) is a unary operator in relational
algebra that performs a selection operation.
·
It selects those rows or tuples from the relation that
satisfies the selection condition.
·
It is a fundamentals / Basic operator
·
It is a Unary Operator
σ<selection_condition>(R)
·
Select tuples from a relation “Books” where subject is
“database”
σsubject = “database” (Books)
·
Select tuples from a relation “Books” where subject is
“database” and price is “450”
σsubject = “database” ∧ price = “450” (Books)
·
Select tuples
from a relation “Books” where subject is “database” and
price is “450” or have a publication year after 2010
σsubject = “database” ∧ price =
“450” ∨
year >”2010″ (Books)
Important
Points-
Point-01:
·
We may use logical operators like ∧ , ∨ , ! and relational operators
like = , ≠ , > , < , <= , >= with the selection
condition.
Point-02:
·
Selection operator only selects the required tuples
according to the selection condition.
·
It does not display the selected tuples.
·
To display the selected tuples, projection operator is
used.
Point-03:
·
Selection operator always selects the entire tuple. It
can not select a section or part of a tuple.
Point-04:
·
Selection operator is commutative in nature i.e.
σ A ∧ B (R)
= σ B ∧ A (R)
OR
σ B (σ A(R))
= σ A (σ B(R))
Point-05:
·
Degree of the relation from a selection operation is same
as degree of the input relation.
Point-06:
· The number of rows returned by a selection operation is obviously less than or equal to the number of rows in the original table.
·
Minimum Cardinality = 0
·
Maximum Cardinality = |R|
Friday, 16 February 2024
Class 11 CS Practical - 52
52 - Write a program in Python to find the mean of numeric values stored in the given tuple.
T=eval(input("Enter a numeric tuple: "))
print("Tuple is: \n", T)
mean=sum(T)/len(T)
print("Mean of the numeric values of the tuple = ",mean)
Output:
Enter a numeric tuple: (2, 8, 9, 6, 5)
Tuple is:
(2, 8, 9, 6, 5)
Mean of the numeric values of the tuple = 6.0
Thursday, 15 February 2024
Class 11 CS Practical - 51
51 - Write a program in Python to count the frequency of all elements of a tuple
T= (10, 20, 30, 20, 40, 30, 20, 50)
print("Tuple is: ",T)
Uni= ()
for i in T:
if i not in Uni:
Uni = Uni + (i,)
for i in Uni:
print(i,":",T.count(i))
Output:
Tuple is: (10, 20, 30, 20, 40, 30, 20, 50)
10 : 1
20 : 3
30 : 2
40 : 1
50 : 1
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