使用展平结果加入查询

Question

我有以下实体

AddressType只是一个枚举字段，用于定义电子邮件是个人/工作/其他。

是否可以执行返回展平结果的查询，如下例中的结果？

CustomerID    Full Name       Personal Email         Work Email
-----------   --------------  -----------------      -----------------------
1             John Doe        johndoe@hotmail.com    john.doe@company.com

Answer 1

select c.CustomerID, 
    c.FullName as [Full Name], 
    epersonal.AddressText as [Personal Email], 
    ework.AddressText as [Work Email]
from Customer c
left outer join Email epersonal on c.CustomerID = epersonal.CustomerID 
    and epersonal.AddressType = 'personal'
left outer join Email ework on c.CustomerID = ework.CustomerID 
    and epersonal.AddressType = 'work'

Answer 2

两个主要选择：

1）选择它为典型（有两行，每封电子邮件一行），然后使用pivot运算符展平。

pivot的例子（我把它称为一个例子，因为我在记事本中写了它。可能有点错误，但它应该指出正确的方法）：

select
  CustomerID,
  FullName
  [1] as WorkEmail,
  [2] as HomeEmail
from
  (select 
    c.CustomerID, c.FullName, e.AddressText, e.AddressType
   from 
     Customer c
     join emails e on e.CustomerID = c.CustomerID) as Source
pivot (
  AddressText
  FOR AddressType in ([1], [2])
)

2）两次加入电子邮件表，每种类型的地址一次。建议外联接，如果缺少一个你仍然得到另一个。