Question

我正在尝试从两个表中获取和联接来自SQL Server的信息。

假设我有这两个表：

表Resources：

ID    Resource
---------------
1     Alex
2     Dennis
3     Diane

表Projects：

Project      Manager    Leader    Engineer
------------------------------------------
pro_18001    1          2         3

我想得到以下结果：

Project        Manager    Leader    Engineer
---------------------------------------------
pro_18001      Alex       Dennis    Diane

我认为SQL查询应该类似于：

SELECT
  [Projects].[Project]
  [Resources].[Resource]
  [Resources].[Resource]
  [Resources].[Resource]
FROM
  [Projects]
LEFT JOIN
  [Resources] ON
    [Projects].[Manager] = [Resources].[ID] AND
    [Projects].[Leader] = [Resources].[ID] AND
    [Projects].[Engineer] = [Resources].[ID]
WHERE
  [Project].[Projects] = 'pro_18001'

但是我确定下面的部分是不正确的，因为同一列有3次，并且联接可能无法以正确的顺序选择正确的列...

SELECT
  [Projects].[Project]
  [Resources].[Resource]
  [Resources].[Resource]
  [Resources].[Resource]

那么如何在正确的位置多次连接同一列？

Answer 1

您需要为每个列添加左连接3，而不是1：

SELECT
  [Projects].[Project],
  Manager.[Resource] AS Manager,
  Leader.[Resource] AS Leader,
  Engineer.[Resource] AS Engineer
FROM [Projects]
LEFT JOIN [Resources] Manager
       ON [Projects].[Manager] = Manager.[ID]
LEFT JOIN  [Resources] Leader
       ON [Projects].[Leader] = Leader.[ID]
LEFT JOIN [Resources] Engineer
       ON [Projects].[Engineer] = Engineer.[ID]
WHERE [Project].[Projects] = 'pro_18001'

Answer 2

您可以尝试以下查询查询所需的结果

create table #Resources (Id int, Resources Varchar(20))
insert into #Resources 
values(1, 'Alex'), (2, 'Dennis'), ('3', 'Diane')

Create table #Projects (Project Varchar(20), Manager INT, Leader INT, Engineer INT)
insert into #Projects Values ('pro_18001', 1, 2, 3)

SELECT #Projects.Project,Manager.Resources as Manager,Leader.Resources as Leader, Engineer.Resources as Engineer FROM #Projects
INNER JOIN #Resources as Manager on #Projects.Manager = Manager.Id
INNER JOIN #Resources as Leader on #Projects.Leader = Leader.Id
INNER JOIN #Resources as Engineer on #Projects.Engineer = Engineer.Id

DROP TABLE #Resources
DROP TABLE #Projects

输出如下

Project     Manager Leader  Engineer
pro_18001   Alex    Dennis  Diane

希望这会对您有所帮助。

Answer 3

SELECT
    p.project_name,
    r.name AS manager,
    r1.name AS leader,
    r2.name AS enginener
FROM
    projects p
INNER JOIN resources r ON
    p.manager = r.id
INNER JOIN resources r1 ON
    p.leader = r1.id
INNER JOIN resources r2 ON
    p.engineer = r2.id

SQL Server：多次连接同一列

3 个答案: