Join 2 tables on different columns based on condition - sql

Question

I have two tables, tbl and tbl2 I have to join. Columns of tbl are irrelevant. Here is the tructure of tbl2:
CREATE TABLE tbl2 (
a VARCHAR(10)
, b VARCHAR(10)
, c VARCHAR(10)
, d VARCHAR(10)
, e VARCHAR(10)
);
What I need to do is apply the following join:
if there are several rows with the same a, b and c LEFT JOIN them on a, b, c and d.
else if there are several rows with the same a and b and they are not in the set mentioned above LEFT JOIN on a, b and c
else LEFT JOIN remaining rows from tbl2 on a and b.
I have been thinking what is the best way to achieve the following. tbl2 can be modified since it is only used for the purpose of this query.
Do you have any ideas what is the most efficient way of achieving this?
EDIT:
By 'having the same a, b and c' I meant something like this:
SELECT a, b, c FROM tbl2 GROUP BY a, b ,c HAVING COUNT(*) > 1

Answer 1

WITH tab_a AS
(
SELECT t2.a
, t2.b
, t2.c
, t2.d
, t2.e
, CASE WHEN t1.c = t2.c THEN 1 ELSE 0 END +
CASE WHEN t1.d = t2.d THEN 1 ELSE 0 END AS other_two
FROM tbl t1
LEFT JOIN tbl2 t2
ON t1.a = t2.a
AND t1.b = t2.b
)
SELECT a
, b
, c
, d
, e
FROM tab_a
WHERE other_two = (SELECT MAX(other_two) FROM tab_a);

Answer 2

put these three queries in if else condition. If you want them all then UNION these three.
select *
from tbl t1 left outer join tbl2 t2
on (t1.a=t2.a and t1.b=t2.b and t1.c=t2.c and t1.d=t2.d)
where t2.a=t2.b and t2.b=t2.c;
select *
from tbl t1 left outer join tbl2 t2
on (t1.a=t2.a and t1.b=t2.b and t1.c=t2.c )
where t2.a=t2.b and t2.b<>t2.c;
select *
from tbl t1 left outer join tbl2 t2
on (t1.a=t2.a and t1.b=t2.b)
where t2.a<>t2.b and t2.b<>t2.c;
or try this:
select *
from tbl t1 left outer join tbl2 t2
on (
(t1.a=t2.a and t1.b=t2.b and t1.c=t2.c and t1.d=t2.d and t2.a=t2.b and t2.b=t2.c)
or
(t1.a=t2.a and t1.b=t2.b and t1.c=t2.c and t2.a=t2.b and t2.b<>t2.c)
or
(t1.a=t2.a and t1.b=t2.b and t2.a<>t2.b and t2.b<>t2.c) );