How to update an entire table by getting the values from another table?

Hi. I need to run an update query on a table, and need some help on
how to do it.


I have a table A(id integer primary key, prod_id integer, Y1 integer,
Y2 integer, ... Y14 integer)  and a table B (a integer references A, x
integer,unique(a) );

and now I should set (for each row in A)  prod_id to have the value
that is found in table B

How do I write a statement that does that?