Retrieve the extents for every category

The need is to retrieve the BOX extent of every group of data.
The gropu are on field ANNO.

The solution is a group by, an Union and the Extent.
So:

select ANNO, ST_Union(GEOMETRY) from the_schema.the_table group by ANNO order by ANNO;

it work.

So I complete composing the string I need.

-----
select
    b.anno,
    'EXTENT ' || ST_XMin(b.extent)::integer - 100 || ' ' || ST_YMin(b.extent)::integer - 100 || ' ' || ST_XMax(b.extent)::integer + 100 || ' ' || ST_YMax(b.extent)::integer + 100 as extent
from
    (
        select anno, ST_Extent (geometry) as extent from the_schema.the_table group by anno
    ) as b
order by
    b.anno
;
-----

its all.