Problem with Nested Inserts

[Juan Javaloyes]

This is my problem. I have two tables (let's called "a" and "b"). I need to load all the columns (specifically, 15.000) "a4", "a5" and "a6" from "a" with data from columns "b3", "b6" and "b7" from "b" (being same datatype), where "acode" (on "a") = "bcode" (on "b"). I did this (the closest solution I tried, at least):

insert into a (a4, a5, a6)
select b3, b6, b7
from b
where bcode = (select acode from a)

The error says "Subquery returns more than 1 row"

I must say all columns on "a" (except those I want to be loaded) are already loaded with data, included (of course!) "acode"
I know this may had a simple solution, but I tried everything, may I be just tired?