Self Doubt Relational Algebra

Question

Let R = {a,b,c} and S = {c}, what does the highlighted portion mean ?

Comments

are 'R' and 'S' the sets.??????

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Highlited portion is Generalized Projection operator which is nothing but an extension of Projection operator. 

In this simple arithmetic operations can be used in projection list on attributes

R-S = {a,b}, S = {c}

So a,b,c attributes are projected from relation r. 

Answer 1

R and S are relation schemas. in book small characters represent relation instance such as r and s, while capital letters represent relation schema ( attributes of a relation).

R = {A, B, C} S ={C}

R - S = {A,B}

$\prod$_{R-S, S}(r)    means $\prod$ _A,B,C(r)

PS: The R.A expression given in the question is for 'division' operation, generally used for "All".

Answer 2

R-S represents DIVISION operation

Answer 3

R-S could also mean extended relation algebra operation. R-S equals a new column that is result of subtraction of values of R and S.