projection

Question

The Project operation $(\prod)$ in relational algebra:

(i) Is used for eliminating duplicate rows automatically from the output.

(ii). Is used to filter columns.

(iii). Is a unary operator.

(iv). has degree of output relation same as input relation.

(A). (i). and (iii)

(B). (ii) and (v)

(C). (i) and (ii)

(D). (iv) only

Comments

All the 4 are correct na ???

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iv is false

degree means no of attributes /columns in relation

so  degree after projection can be less than or equal to degree of relatiomn

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but the answer given is 4. Are they asking about which one is false ??

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in B its written (ii) and (v) . where they have meant (ii) and (iv). So I think the given answer is wrong. It  must be every option except (iv).

Answer 1

The Project operation in relational algebra : 
a. Is used to filter columns : True
b. Is a unary operator : True
c. has degree of output relation same as input relation : False
d.  Is used for eliminating duplicate rows automatically from the output : True

Answer 2

project operation corresponds to select operation in sql except the fact that in sql select doesnt remove duplicates cause it is expensive but project does remove duplicate so Option A is true..

It also filter out columns ..example ->consider a realtion with attribute A ,B,C ..project can be used to o/p the  column A..so Option B is also true

Unary operator are those that takes only one realtion as i/p ..project,select and rename are unary operator  so option C is true

Degree of a relation means no of attributes but project can be used to o/p less number of columns as well .. so D is false