하위 쿼리 각 상위 ID에 대한 최신 항목을 반환합니다.

Question

문서 항목이있는 부모 테이블이 있으며 사용자가 문서 중 하나에 액세스 할 때마다 감사 항목을 기록하는 기록 테이블이 있습니다.

결과 세트에서 반환 된 각 문서에 액세스하기 위해 최신 사용자 ID와 함께 문서 목록 (다양한 기준별로 필터링)을 반환하기 위해 검색 쿼리를 작성합니다.

따라서

    DOCUMENTS
    ID | NAME
    1  | Document 1
    2  | Document 2
    3  | Document 3
    4  | Document 4
    5  | Document 5

    HISTORY
    DOC_ID | USER_ID | TIMESTAMP
    1      | 12345   | TODAY
    1      | 11111   | IN THE PAST
    1      | 11111   | IN THE PAST
    1      | 12345   | IN THE PAST
    2      | 11111   | TODAY
    2      | 12345   | IN THE PAST
    3      | 12345   | IN THE PAST

나는 내 검색에서 돌아올 것을 찾고있을 것이다.

    ID | NAME       | LAST_USER_ID
    1  | Document 1 | 12345
    2  | Document 2 | 11111
    3  | Document 3 | 12345
    4  | Document 4 | 
    5  | Document 5 | 

하나의 SQL 쿼리와 두 테이블 사이의 조인으로 쉽게이 작업을 수행 할 수 있습니까?

Answer 1

Revising what Andy White produced, and replacing square brackets (MS SQL Server notation) with DB2 (and ISO standard SQL) "delimited identifiers":

SELECT d.id, d.name, h.last_user_id
    FROM Documents d LEFT JOIN
         (SELECT r.doc_id AS id, user_id AS last_user_id
              FROM History r JOIN
                   (SELECT doc_id, MAX("timestamp") AS "timestamp"
                        FROM History
                        GROUP BY doc_id
                   ) AS l
                   ON  r."timestamp" = l."timestamp"
                   AND r.doc_id      = l.doc_id
         ) AS h
         ON d.id = h.id

I'm not absolutely sure whether "timestamp" or "TIMESTAMP" is correct - probably the latter.

The advantage of this is that it replaces the inner correlated sub-query in Andy's version with a simpler non-correlated sub-query, which has the potential to be (radically?) more efficient.

Answer 2

I couldn't get the "HAVING MAX(TIMESTAMP)" to run in SQL Server - I guess having requires a boolean expression like "having max(TIMESTAMP) > 2009-03-05" or something, which doesn't apply in this case. (I might be doing something wrong...)

Here is something that seems to work - note the join has 2 conditions (not sure if this is good or not):

select
    d.ID,
    d.NAME,
    h."USER_ID" as "LAST_USER_ID"
from Documents d
left join History h
    on d.ID = h.DOC_ID
    and h."TIMESTAMP" =
    (
        select max("TIMESTAMP")
        from "HISTORY"
        where "DOC_ID" = d.ID
    )

Answer 3

This doesn't use a join, but for some queries like this I like to inline the select for the field. If you want to catch the situation when no user has accessed you can wrap it with an NVL().

select a.ID, a.NAME,
(select x.user_id
 from HISTORY x
 where x.doc_id = a.id
   and x.timestamp = (select max(x1.timestamp)
                      from HISTORY x1
                      where x1.doc_id = x.doc_id)) as LAST_USER_ID
from DOCUMENTS a
where <your criteria here>

Answer 4

I think it should be something like this:

SELECT ID, Name,  b.USER_ID as LAST_USER_ID
FROM DOCUMENTS a LEFT JOIN
    ( SELECT DOC_ID, USER_ID 
          FROM HISTORY
              GROUP BY DOC_ID, USER_ID
              HAVING MAX( TIMESTAMP )) as b
    ON a.ID = b.DOC_ID

this might work also:

SELECT ID, Name,  b.USER_ID as LAST_USER_ID
FROM DOCUMENTS a 
  LEFT JOIN HISTORY b ON a.ID = b.DOC_ID
GROUP BY DOC_ID, USER_ID
HAVING MAX( TIMESTAMP )

Answer 5

Select ID, Name, User_ID
From Documents Left Outer Join
History a on ID = DOC_ID
Where ( TimeStamp = ( Select Max(TimeStamp)
                      From History b
                      Where a.DOC_ID = b.DOC_ID ) OR
        TimeStamp Is NULL )  /* this accomodates the Left */