database

Question

What is the correct interpretion of this query

Comments

Is ESSN(and not SSN) not the second attribute in Employee Relation Schema?

---

Anyway, Result will contain the name of employees who are managers but no one is dependent on them. (Employess with SSN, who are having entry in MGRSSN of department but Not having in ESSN of Dependent)

---

m not getting the C<-----------A-B part pls explainby taking example

---

Well think of it as:

Let say, A has IDs of All the employees who are managers, while B has IDs of all the employees who has someone dependent upon them. Then C = A-B, will simply contain all the managers upon whom no one is dependent.

Answer 1

A, B and C are temprary variables

All the MGRSSN will be stored in A from Department table under the column name SSN.
All the ESSN will be stored in B from Dependent table under the column name SSN. So that the set difference can be calculated further.

C <-- A-B 

C will store those MGRSSN who don't have any dependent.

There is natural join of employee with C, it means the names of those employees who are manager and don't have any dependent will be stored in the Result variable.