join on
  CASE FIELD1
   When 1 THEN FIELD 2 = FIELD3
   When 2 THEN FIELD 4 = FIELD5
  END

.... join on  (field1=1 AND field2=field3) OR (field1=2 AND field4=field5)

select ...
from ...
join ...
   on  (field1=1 AND field2=field3)
union
select ...
from ...
join ...
   on  (field1=2 AND field4=field5)

select * from table as x where x.field1 = 1 AND x.field2 = x.field3
union [ALL]
select * from table as y where x.field1 = 2 AND y.field4 = y.field5

from A left outer join
     B b1
     on a.field2 = b2.field3 left outer join
     B b2
     on a.field4 = b2.field5

SELECT *
FROM data_parent
WHERE pid =
CASE
WHEN pid =1
THEN (
SELECT id
FROM data_parent, test
WHERE data_parent.pid = test.id
)