SELECT database_state_desc from sys.dm_hadr_database_replica_states

SELECT 
CASE  database_state_desc 
WHEN 'ONLINE' then 'Primary'
end
from sys.dm_hadr_database_replica_states

null

SELECT  *
CASE  database_state_desc 
WHEN 'ONLINE' then 'Primary'
end
from sys.dm_hadr_database_replica_states

SELECT  *
,
CASE  
WHEN database_state_desc  = 'ONLINE' then 'Primary'
When database_state_desc is null Then Secondary 
else database_state_desc end
from sys.dm_hadr_database_replica_states

When database_state_desc is null Then 'Secondary'

select str_
     , replace(isnull(str_,'Secondary'),'Online','Primary') as new_str
  from (
         select cast('Online' as varchar(10)) as str_
         union all
         select NULL
       ) v