select deocde(a.str,null,decode(b.str,null,c.str,b.str),a.str) str,c.integer from ..................
SELECT a.str || ' ' || b.str || ' ' || c.str as str, a.intege,b.intege,c.intege FROM t_testa a FULL JOIN t_testb b ON a.str = b.str FULL JOIN t_testb c ON c.str = c.str
SELECT a.str || ' ' || b.str || ' ' || c.str as str, a.intege,b.intege,c.intege
FROM t_testa a
FULL JOIN t_testb b
ON a.str = b.str
FULL JOIN t_testb c
ON c.str = c.str