Mysql 选择主要或次佳记录

我有三张桌子，上面有联系人的信息和地址

Contact
  id
  name
  dob
  etc...

Address
 line1
 line2
 postcode
 etc...

ContactAddress
  contact_id
  address_id
  primary

我正试图找出如何获得联系人列表和一个地址。此地址应为主地址（如果主地址设置为true），如果有一个可用数据，则为辅助地址，如果两个数据都不可用，则为空白数据

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显然，在ContactAddress上使用group by会为每个联系人提供一个地址，但我如何将这些地址标记为primary（主地址）的优先级。

此查询应该可以做到这一点

SELECT c.Id, c.Name, c.DOB, a.Line1, a.Line2, a.Postcode
FROM Contact c
INNER JOIN ContactAddress ca ON c.Id = ca.Contact_Id
INNER JOIN [Address] a ON ca.Address_Id = a.Id
INNER JOIN
    (SELECT c.Id AS Contact_Id,
        (SELECT TOP 1 Address_Id FROM ContactAddress WHERE Contact_Id=c.Id ORDER BY [Primary] DESC) AS Address_Id
    FROM Contact c) x
ON ca.Contact_Id = x.Contact_Id and ca.Address_Id = x.Address_Id

UNION

SELECT c.Id, c.Name, c.DOB, null, null, null
FROM Contact c 
WHERE NOT EXISTS (SELECT * FROM ContactAddress WHERE Contact_Id=c.Id)