Updating a field by the order of onother...

[Roberto Cano Ramos]

Hi!
I have the following table

Table name "control"
ID:identity
ID_MEMBER:integer
ORDER_NUM:integer

I need to update the whole table. Seetting the field ORDER_NUM to the position of the order by ID_MEMBER....it is dificult to explain....
so here goes an example

let say that I have the following values

ID ID_MEMBER ORDER_NUM
----------------------------------------
1 1 ?
2 5 ?
3 2 ?

I need to update the table to:
ID ID_MEMBER ORDER_NUM
----------------------------------------
1 1 1
2 5 3
3 2 2

It should be simple but I do not know how.....
set @t=1;
update control set @t=@t+1,ORDER_NUM=@t
of course this does not work......

As you probably guess this table is used to control the memberships orders of another table that has all the deltails of the member....The spanish law obliges you to keep a numeric order without gaps.
I need call this update every time someone adds a member or disables it membership or activates its.