在其中一个连接上选择具有最大值的记录

Question

我有三个相关的 table 并且需要 select 行显示来自两个 table 的数据基于第三个的值（序列号）。我只对序列号的最大值感兴趣。我已经尝试了在 Whosebug 上建议的多种解决方案，但我仍然无法解决这个问题。

我的 table 的示例代码，所有值的直接 SELECT 可在此处获得：http://sqlfiddle.com/#!6/6b8f7/4/0

我的最终目标是获得这样的 table：

reference   groupname   serialnum
C:123       Group2      3
C:125       Group1      4
C:126       Group1      1

使用 LIMIT 订购似乎无效。知道如何解决这个问题吗？

示例数据的 DDL + DML：

CREATE TABLE pm_process
    ([pm_guid] int, [Descr] varchar(4), [usr_newref] varchar(5))
;

INSERT INTO pm_process
    ([pm_guid], [Descr], [usr_newref])
VALUES
    (11111, 'aaaa', 'C:123'),
    (22222, 'bbbb', 'C:125'),
    (33333, 'cccc', 'C:126')
;


CREATE TABLE tps_group
    ([tps_title] varchar(6), [tps_guid] int)
;

INSERT INTO tps_group
    ([tps_title], [tps_guid])
VALUES
    ('Group1', 99999),
    ('Group2', 88888)
;


CREATE TABLE pm_process_assignment
    ([pm_group_guid] int, [pm_process_guid] int, [pm_serial_number] int)
;

INSERT INTO pm_process_assignment
    ([pm_group_guid], [pm_process_guid], [pm_serial_number])
VALUES
    (99999, 11111, 1),
    (99999, 11111, 2),
    (88888, 11111, 3),
    (88888, 22222, 1),
    (99999, 22222, 2),
    (88888, 22222, 3),
    (99999, 22222, 4),
    (99999, 33333, 1)
;

Answer 1

您可以使用 ROW_NUMBER() 在每个 reference 分区中查找具有最大 serialnum 的记录。然后，在外部查询中，select 只有这些记录：

SELECT reference, groupname, serialnum
FROM (
SELECT
  pm_process.usr_newref as reference,
  pm_assignment_group.tps_title as groupname,
  process_assignments.pm_serial_number as serialnum,
  ROW_NUMBER() OVER (PARTITION BY pm_process.usr_newref 
                     ORDER BY process_assignments.pm_serial_number DESC) AS rn
FROM
  tps_group  pm_assignment_group 
  RIGHT OUTER JOIN pm_process_assignment  process_assignments 
    ON (pm_assignment_group.tps_guid=process_assignments.pm_group_guid)
  RIGHT OUTER JOIN pm_process 
    ON (process_assignments.pm_process_guid=pm_process.pm_guid)
) t
WHERE t.rn = 1

SQL Fiddle Demo

Answer 2

在 SQL 服务器中，最简单的方法可能是使用 APPLY:

SELECT p.usr_newref as reference,
       pag.tps_title as groupname,
       pag.pm_serial_number as serialnum
FROM pm_process p OUTER APPLY
     (SELECT TOP 1 pa.pm_serial_number, g.tps_title
      FROM pm_process_assignment pa JOIN
           tps_group g
           ON g.tps_guid = pa.pm_group_guid
      WHERE pa.pm_process_guid = p.pm_guid
      ORDER BY pm_serial_number DESC
     ) pag

Here 是 SQL Fiddle.

Answer 3

使用 RANK 和 CTE。它会处理多个 pm_serial_number 具有相同值的情况。

WITH RankTable AS
(

    SELECT *, RANK() OVER (
        PARTITION BY usr_newref 
        ORDER BY pm_serial_number DESC) AS R
    FROM pm_process
    LEFT JOIN pm_process_assignment 
        ON pm_process_assignment.pm_process_guid = pm_process.pm_guid
    LEFT JOIN tps_group 
        ON tps_group.tps_guid = pm_process_assignment.pm_group_guid
)
SELECT usr_newref, tps_title, pm_serial_number 
FROM RankTable 
WHERE RankTable.R = 1

在其中一个连接上选择具有最大值的记录

Selecting a record with max value on one of the joins

sql

sql-server

greatest-n-per-group