tuple relational calclus

Question

A database have relation schema Student(s_id,s_name,dept_id) and dept(dep_name,dept_id). Note that the dept_id can be permitted to be null in relation student .consider the following queries on the database expression in tuple relational calclus.

{t| ∃u ∈ student(t[s_name]=u[s_name] ∧ ∀v ∈ DEPT(t[Dept_Id] ≠ v[DeptId]))}

is this expression is safe.

Comments

@sachin! no it never gives infinite tuples. It is bounded to that table only. What @Anu007 saying is exactly correct.

---

s_id	s_name	deptid
4589	rohan	5
7895	rohit	3
4789	ram	null
6789	sundar	6

Dept

dept name	dept id
physics	5
chemistry	6
mathematics	3

according to {t | ∃u ∈ student(t[s_Name] = u[s_Name] ∧∀v ∈ DEPT(t[DeptId] ≠ v[DeptId]))}

 can be collection of tuple

emp name	deptid
rohan	8
rohan	9
rohan	10
rohan	11
rohan	12

similarly it can take infinite tuple not for rohan only but for all name inside emp

then it is how a safe expression.

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No no this query is not executing this way. It returns student name of that student who does not belong to any given dept. whenever dept_id of student matches with dept_id of dept "for all" returns false and that student name will not be taken in the output.

In this way this query works.