在SQL Server中查詢多個表

Question

我有3個表Manager ， Lead ， Trainee以及它們通過Relation表的Relation 。

所附圖片描繪了帶有行和列的表格：

我需要得到Lead名和Trainee名稱為給定的Manger.Id 。

我從以下查詢中獲得了參數名稱：

select 
    L.Name
from 
    Lead L, Manager M, Relation R 
where 
    R.PrimaryId = M.Id 
    and R.SecondaryId = L.Id 
    and M.Id = 'M101';

我如何獲得學員姓名和線索名稱？

Answer 1

如果使用聯接而不是1992年前的舊語法編寫查詢，則查詢會更清晰：

select  L.Name
from    Manager M
join    Relation R 
on      R.PrimaryId = M.Id
join    Lead L
on      R.SecondaryId = L.Id
where   M.Id = 'M101'

您可以添加兩個新的left join以顯示每個經理的受訓者：

select  L.Name as lead
,       R.Name as trainee
from    Relation ML
join    Lead L
on      ML.SecondaryId = L.Id
left join
        Relation LT
on      LT.PrimaryId = L.Id
left join
        Trainee T
on      LT.SecondaryId = T.Id
where   ML.PrimaryId = 'M101'