[Oracle] How to monitor real SGA memory usage

From something read on the net :

select name, round(sum(mb),1) mb, round(sum(inuse),1) inuse
from (
    select case when name = 'buffer_cache' then 'db_cache_size'
                when name = 'log_buffer' then 'log_buffer'
                else pool end name,
           bytes/1024/1024 mb,
           case when name = 'buffer_cache' then (bytes - (select count(*) from v$bh where status = 'free') *
                                                         (select value from v$parameter where name = 'db_block_size')
                                                 )/1024/1024
                when name <> 'free memory' then bytes/1024/1024
                end inuse
    from v$sgastat
)
group by name;