Oracle 18c

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create table test (factor NUmber(10));
insert into test values(10)
1 rows affected

select * from test


FACTOR


10


SELECT 
   CASE WHEN COUNT(factor) > 1 
   THEN 
      NULL
   ELSE 
     max(factor)
   END
FROM test


CASEWHENCOUNT(FACTOR)>1THENNULLELSEMAX(FACTOR)END


10


SELECT 
   CASE WHEN COUNT(factor) > 1 
   THEN 
      NULL
   ELSE 
     factor
   END
FROM test
ORA-00937: not a single-group group function