Question

I have a table attach with huge data sets, it is a temp table and created by sql:

    id   number   name
    1      X1     name1
    2      X2     name2
    3      X3     name3
    4      X4     name4

table attachment_map

    id    item    attach_id  file_id  versionid
    1      X1       1          100         0
    2      X2       0           0          1

table version

   id    attach_id   
    1      2       

I want to have query to get:

    id   number   name    item
    1      X1     name1    X1
    2      X2     name2    X2
    3      X3     name3
    4      X4     name4

As you can see, the return rows added new column which actually got from table attachment_map, there have three impossibles:

1).attach don’t have item.

2).Have item, by connecting to column attach_id of attachment_map.

2).Have item, by connecting to column attach_id of version.

I wrote a query but having poor performance, executed it slowly and it seems because of union. can everybody tell another way or how I can improvee it? thanks

 WITH tb AS
(SELECT t.*,
 i.item
 FROM attach t,
  attachment_map am,
  version v
 WHERE (am.attach_id = t.attachid
OR (am.file_id      = 0
AND am.version_id   = v.id))
)
SELECT * FROM tb
UNION
SELECT tb.*, NULL FROM tb, attach WHERE tp.id NOT IN (attach.id);

Answer 1

Use two LEFT OUTER JOINs to link the three tables, and then CASE the item fields in the order of priority (first inspect attach, then attachment_map, then version).

SELECT t.*
, case when t.attach_id is null then
    case when am.attach_id is null then
        v.item
    else
        am_attach.item
    end
else
    t.item
end as chosen_item

FROM attach t
left outer join attachment_map am on am.attach_id = t.attachid
left outer join version v on am.version_id   = v.id