How do I do the spatial join?

[Flugan87]

Hello!
I'm trying to select just some buldings within a polygon.
I want to get the polygon in fastkart.ak layer that have the kommunkod = 2085.

I want all data from fastkart.by that are within the polygon.
I'm quite new to sql and can't figuare out how to do this.

I've added a picture with database and my sql so far.

SELECT * as by
(SELECT kommunkod
FROM fastkart."ak" as ak
WHERE kommunkod = '2085')
FROM fastkart."by" by
WHERE ST_Intersects(b y.geom,ak.geom)

Can anyone help me?