database - About query written in Domain Relational Calculus -

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i trying learn domain relational calculus. considering below relations , queries, why can't write for{<c>|< b, l, c, a> ∈ borrow ∧ a>1200} second question? have feeling has whether query safe, how come answer question 1 works? don't know wrong. please explain? many thanks! enter image description here enter image description here source http://www.cs.sfu.ca/coursecentral/354/zaiane/material/notes/chapter3/node18.html#section00142000000000000000 , relations refers here http://www.cs.sfu.ca/coursecentral/354/zaiane/material/notes/chapter3/node8.html#section00121000000000000000

remember, definition of query in domain relational calculus

{<x1, ..., xn> | p(<x1, ..., xn>)}

says variables x1, ..., xn appear left of ‘|’ must free variables in formula p(...). in example, variables b,l,a not bound , remain free. in correct answer, existence quantifier binds them.


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