SELECT A.* 
   ,REPLACE(TEXT, PARAM_NAME ,
    dbms_xmlgen.getxmltype('SELECT '|| A.db_column ||' FROM B WHERE code='''|| A.code||'''' ).Extract('//text()') 
    )
 FROM A 

WITH a AS
(
SELECT 'P1' param_name, 'AAA' db_column, 'P1 P2 P3' text, 'ABCD' code FROM dual
UNION ALL SELECT 'P2', 'BBB', 'P1 P2 P3', 'ABCD' FROM dual
UNION ALL SELECT 'P3', 'CCC', 'P1 P2 P3', 'ABCD' FROM dual
)
, b AS
(
SELECT 'ABCD' code, '111' aaa, '222' bbb, '333' ccc FROM dual
)
-- 어차피 컬럼명은 몇가지로 한정되어 있겠지요...
SELECT a.code
     , a.param_name
     , a.db_column
     , b.aaa, b.bbb, b.ccc
     , a.text
     , REPLACE(a.text, a.param_name
       , DECODE(a.db_column, 'AAA', b.AAA, 'BBB', b.BBB, 'CCC', b.CCC)
       ) text2
  FROM a, b
 WHERE a.code = b.code
;