Use Aggregate Function in UNION ALL result set

How can I use aggregate Functions in UNION ALL Resultset

FOR EXAMPLE

SELECT A,B FROM MyTableUNION ALLSELECT B,C FROM MYAnotherTable

Result Set Would Be

    A  B--------------    1  2    3  4    4  5    6  7

When I tried to obtain MAX(A) it returns 3. I want 6.

When I tried to obtain MAX(B) it returns 4. I want 7.

Other than Max(), Can I obtain another aggregate function which user defined?

For example:

(SELECT TOP 1 A WHERE B=5)

Try this way:

select max(A)from(      SELECT A,B FROM MyTable      UNION ALL      SELECT B,C FROM MYAnotherTable    ) Tab

If the column A is varchar (You said that in the comment below) try this way:

select max(A)from(      SELECT cast(A as int) as A,B FROM MyTable      UNION ALL      SELECT B,C FROM MYAnotherTable    ) Tab

With TOP 1

select max(A)from(      SELECT top 1 cast(A as int) as A,B FROM MyTable      UNION ALL      SELECT B,C FROM MYAnotherTable    ) Tab