SELECT NVL(a.sample_id,'-')
, NVL(a.text_value,'-')
FROM NAIS_all_SAMPLES s
, nais_sample_attributes a
WHERE 
s.sample_id = 200000698 
and s.sample_id=a.sample_id(+) 
and upper(a.name)(+) = 'COMMENTAIRE'

SQL> select * from samples;

 SAMPLE_ID SOME_DATA
---------- ----------
         1
         2
         3
         4

SQL> select * from sample_attributes
  2  ;

 SAMPLE_ID TEXT_NAME       TEXT_VALUE           
---------- --------------- -----------------
         1 COMMENTAIRE     TOP GRADE SAMPLE     
         1 other           something            
         2 COMMENTAIRE                          
         3                                      

SQL> SELECT s.sample_id
  2        ,a.text_value
  3  FROM   samples           s
  4        ,sample_attributes a
  5  WHERE  s.sample_id = a.sample_id(+)
  6  and    upper(a.text_name(+)) = 'COMMENTAIRE';

 SAMPLE_ID TEXT_VALUE
---------- ---------------------------------------
         1 TOP GRADE SAMPLE
         2
         3
         4