How can I find MAX with relational algebra?

Question

Working with databases, how can I find MAX using relational algebra?

Answer 1

lets think we have a relation with an attribute A and values 1,2,3

A

1
2
3

so now..

project A values and rename with A1

A1
1
2
3

again project A values and rename with A2

A2
1
2
3

join this with A2 i.e \join_{A2
so the - Output schema: (A2 integer, A1 integer)

A2

hear always A2 values will be less than A1 because we join like that(a2)

now project A2 the output is like below

A2
1
2

now diff with original attribute

A diff A2

A
1
2
3

 diff

A2
1
2

Output is 3 maximum value