Q41

MCQ2 marks, −0.67

Consider a database that has the relation schemas EMP(EmpId, EmpName, DeptId), and DEPT(DeptName, DeptId). Note that the DeptId can be permitted to be NULL in the relation EMP. Consider the following queries on the database expressed in tuple relational calculus. Which of the above queries are safe? I and II only I and III only II and III only I, II and III

(A)

{ $t$ | $\exists$ u $\in$ EMP(t[EmpName] = u[EmpName] $\wedge$ $\forall$ v $\in$ DEPT(t[DeptId] $\neq$ v[DeptId]))}

(B)

{ $t$ | $\exists$ u $\in$ EMP(t[EmpName] = u[EmpName] $\wedge$ $\exists$ v $\in$ DEPT(t[DeptId] $\neq$ v[DeptId]))}

(C)

{ $t$ | $\exists$ u $\in$ EMP(t[EmpName] = u[EmpName] $\wedge$ $\exists$ v $\in$ DEPT(t[DeptId] $=$ v[DeptId]))}

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