safe query

Question

Is the following query safe?

Consider a database that has the relation schemas EMP(EmpId, EmpName, DeptId), and DEPT(DeptName, DeptId)

1. DEPT_ID in EMP can be null

2.DPET_ID in EMP cannot be null

{t | ∃u ∈ EMP(t[EmpName] = u[EmpName] ∧ ∀v ∈ DEPT(t[DeptId] == v[DeptId]))}

Comments

1. DEPT_ID in EMP can be null- unsafe query
2. DEPT_ID in EMP cannot be null-safe query

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can you please explain a bit? I'm having trouble understanding this.

All I know is that, the tuples returned must be finite for it to be safe.

I don't understand how having a null can fetch me infinite tuples.

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cannot be null always give a finite value

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You are stating this as if it is a fact. Can you explain a bit? Or reference?

Why do you say that "cannot be null" always gives a finite value? How does that prove 'null' doesn't?

Answer 1

i think all are safe .

1 and 2 are SQL query and sql is always safe . third one is tuple relational calculas and it is also safe becz there is no negation operation is perfomed.