MSSQL：左连接三个表中的多个列答案

【问题标题】：MSSQL: left join multiple columns in three tablesMSSQL：左连接三个表中的多个列
【发布时间】：2021-12-31 16:52:58
【问题描述】：

我在 MS SQL Server 中有三个表：

Tenants:
TenantID int,
LastName varchar(50),
FirstName varchar(50)

Intervals:
Value int,
Name varchar(50)

Leases:
LeaseID int,
Interval int,
StartDate date,
EndDate date,
Tenant1 int,
Tenant2 int,
Tenant3 int,
Tenant4 int

Intervals 表有三行：[1,'Monthly']、[2,'BiWeekly'] 和 [3,'Weekly']

我需要一个具有此结果集的查询：

LID Type      Start         End      Tenant1       Tenant2        Tenant3       Tenant4
21  Monthly  12/15/2019  12/15/2020  Sean Murphy   Audrey Moore   Randy Davis
32  Weekly   06/01/2018              Pete Higgins  Kathy Higgins  
35  Monthly  08/01/2019  10/31/2020  Andy Stacke   Valerie Stacke
44  Monthly  01/01/2021              Pete O'Toole  Martha White

暂时先不管名字+姓氏列，我第一次尝试成功地使用了两个左连接：

select L.LeaseID, I.Name as Type, L.StartDate, L.EndDate, T.LastName as Tenant1 
from Leases L 
left join Intervaltypes I on I.value=L.Interval 
left join Tenants T on T.TID = L.Tenant1

并返回：

LID Type      Start         End      Tenant1 
21  Monthly  12/15/2019  12/15/2020  Murphy  
32  Weekly   06/01/2018              Higgins
35  Monthly  08/01/2019  10/31/2020  Stacke
44  Monthly  01/01/2021              O'Toole

一个好的开始，但我似乎无法从租户表中获得最多 4 个唯一名称，我想在租户列中组合 (firstname+lastname)。

我为同一个表（租户）尝试了多个左连接，并在一个连接上尝试了 ORing 条件；他们在直觉上似乎都是错误的。

select L.lid,L.ruid, I.Name, T.LastName as Tenant1, T.LastName as Tenant2 from Leases L 
left join Intervaltypes I on I.value=L.Interval 
left join Tenants T on T.TID = L.Ten1
left join Tenants T on T.TID = L.Ten2

select L.lid,L.ruid, I.Name, T.LastName as Tenant1, T.Lastname as Tenant2 from Leases L 
left join Intervaltypes I on I.value=L.Interval 
left join Tenants T on T.TID = L.Ten1 or T.TID = L.Ten2

但它们都在两列中返回相同的姓氏。

【问题讨论】：

标签： sql left-join

【解决方案1】：

在这里没有网络但我相信你想要这样的东西：

select 
    L.LeaseID AS 'LID', 
    I.Name AS 'Type', 
    L.StartDate AS 'Start', 
    L.EndDate AS 'End', 
    CONCAT(T1.FirstName, ' ', T1.LastName) AS 'Tenant1', 
    CONCAT(T2.FirstName, ' ', T2.LastName) AS 'Tenant2', 
    CONCAT(T3.FirstName, ' ', T3.LastName) AS 'Tenant3', 
    CONCAT(T4.FirstName, ' ', T4.LastName) AS 'Tenant4'
from Leases L
left join Intervals I ON L.Interval = I.Value
left join Tenants T1 ON L.Tenant1 = T1.TenantID
left join Tenants T2 ON L.Tenant2 = T2.TenantID
left join Tenants T3 ON L.Tenant3 = T3.TenantID
left join Tenants T4 ON L.Tenant4 = T4.TenantID

【讨论】：

【解决方案2】：

试试这个

select
  L.lid
  ,L.ruid
  , I.Name
  , T1.LastName as Tenant1
  , T2.LastName as Tenant2 
from Leases L 
left join Intervaltypes I on I.value=L.Interval 
left join Tenants T1 on T1.TID = L.Ten1
left join Tenants T2 on T2.TID = L.Ten2

【讨论】：

阿赫。很简单，很好用，谢谢