How to find the maximum in relational algebra?

Question

This is my database course's homework. We have this relation. Trained(A, B) A trained B

How can I figure out who has trained the most people by using relational algebra or it cannot be expressed in relational algebra? for example, (John, Kenny) (John, Kei) (John, Cohen) (Willson, John) (Kenny, Peter) (Ho, Tina)

In these tables, the answer obviously is John.But is it possible to express in relational algebra?

Answer 1

You use GROUP and HAVING. I don't know how your class does notation, but in mine we do something like this:

SELECT A FROM Trained GROUP BY A HAVING COUNT(A)

I'm not sure if this is correct, but it should be similar.

Answer 2

Yes, it is possible to express it in relational algebra (RA).

The following line groups column A by COUNT(A). And plots COUNT(A) as 'frequencyOf_A'.

γ A; COUNT(A)→frequencyOf_A (Trained)

Resulting in this:

Trained.A   frequencyOf_A
John        3
Willson     1
Kenny       1
Ho          1

I used this RA engine. And your relation is as follows:

Trained = {A:string, B:string
'John'     , 'Kenny'
'John'     , 'Kei'
'John'     , 'Cohen'
'Willson'  , 'John'
'Kenny'    , 'Peter'
'Ho'       , 'Tina'
}