仅当第一次连接没有结果时才连接表

Question

I have 5 tables A, B, C, D, E.

The primary key of A is a foreign key in B, C, D, E. They don't have their own primary key.

I have to implement a certain logic where

if ("A JOIN B JOIN C" IS NOT NULL)
    return result of "A JOIN B JOIN C"
else if ("A JOIN C" IS NOT NULL)
    return result of "A JOIN C"
else if ("A JOIN D" IS NOT NULL)
    return result of "A JOIN D"
else if ("A JOIN E" IS NOT NULL)
    return result of "A JOIN E"
else return A

where A.primarykey = <some value>

I have to implement this in a single sql and not a stored proc. I can't think of a way to achieve this. I use DB2. I would really appreciate if you could give some directions on how to proceed.

Answer 1

You could do something like this:

select
  case 
     when B.key is not null and C.key is not null then 'A+B+C'
     when C.key is not null then 'A+C'
     when D.key is not null then 'A+D'
     when E.key is not null then 'A+E'
  end
from A
left join B on B.key = A.key
left join C on C.key = A.key
left join D on D.key = A.key
left join E on E.key = A.key
left join F on F.key = A.key

The case statement will stop evaluating further as soon as a when condition is met, so effectively you've got an if-else if block.

I am just returning string values in the example, but you could return field values as well, depending on your needs, eg:

  case 
     when B.key is not null and C.key is not null then b.f1
     when C.key is not null then c.f1
     when D.key is not null then d.f1
     when E.key is not null then e.f1
  end as 'f1'
  ,
  case 
     when B.key is not null and C.key is not null then b.f2
     when C.key is not null then c.f2
     when D.key is not null then d.f2
     when E.key is not null then e.f2
  end as 'f2'

Answer 2

You want LEFT JOIN s with with some additional logic to handle the "else" part of what you are doing:

select . . .
from A left join
     B 
     on B.key = A.key left join
     C
     on C.key = A.key left join
     C c2
     on c2.key = A.key and
        c.key is null left join  -- first attempt failed
     D
     on D.key = A.key and
        c.key is null and
        c2.key is null left join  -- earlier joins failed
     E
     on e.key = A.key and
        c.key is null and
        c2.key is null and
        d.key is null

Answer 3

Try the following. You may edit input data to check the result.

WITH 
  A (ID, AV) AS (VALUES (1, 'A1'))
, B (ID, BV) AS (VALUES 
(1, 'B1'), (1, 'B2')
--(2, 'B1'), (2, 'B2')
) , C (ID, CV) AS (VALUES 
(1, 'C1'), (1, 'C2'), (1, 'C3')
--(2, 'C1'), (2, 'C2'), (2, 'C3')
), D (ID, DV) AS (VALUES 
(1, 'D1'), (1, 'D2')
--(2, 'D1'), (2, 'D2')
) , E (ID, EV) AS (VALUES 
(1, 'E1'), (1, 'E2'), (1, 'E3')
--(2, 'E1'), (2, 'E2'), (2, 'E3')
)
SELECT A.ID, A.AV, B.BV, C.CV, C1.CV AS CV1, D.DV, E.EV
, 
CASE
  WHEN B.ID  IS NOT NULL THEN 'A+B+C'
  WHEN C1.ID IS NOT NULL THEN 'A+C'
  WHEN D.ID  IS NOT NULL THEN 'A+D'
  WHEN E.ID  IS NOT NULL THEN 'A+E'
  ELSE 'A'
END JOIN_PATH
FROM A
LEFT JOIN 
(
B 
JOIN C ON C.ID=B.ID
)              ON B.ID  = A.ID 
LEFT JOIN C C1 ON C1.ID = A.ID AND B.ID IS NULL
LEFT JOIN D    ON D.ID  = A.ID AND B.ID IS NULL AND C1.ID IS NULL 
LEFT JOIN E    ON E.ID  = A.ID AND B.ID IS NULL AND C1.ID IS NULL AND D.ID IS NULL;