Checking current SGA usage

select round(used.bytes /1024/1024 ,2) used_mb
, round(free.bytes /1024/1024 ,2) free_mb
, round(tot.bytes /1024/1024 ,2) total_mb
from (select sum(bytes) bytes
from v$sgastat
where name != 'free memory') used
, (select sum(bytes) bytes
from v$sgastat
where name = 'free memory') free
, (select sum(bytes) bytes
from v$sgastat) tot


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select name,
round(sum(mb),1) mb,
round(sum(inuse),1) inuse,
round(sum(free),1) free
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 <> 'free memory'
then bytes/1024/1024
end inuse,
case when name = 'free memory'
then bytes/1024/1024
end free
from v$sgastat
)
group by name;