select GEOM.STEnvelope() from table

select min(p0.STX) as x1,min(p0.STY) as y1,MAX(p1.STX) as x2,MAX(p1.STY) as y2 from (
  select bbox.STPointN(1) as p0,bbox.STPointN(3)as p1 from (
    select  GEOM.STEnvelope() as bbox from (
      select ... from table
    )
  )
)

select geometry::EnvelopeAggregate(GEOM) from table