SQL - LEFT JOIN多个条件 - 优先级

Question

我有2个表,其结构与此类似:

表:用户

fields:id,active_office_address_id(这可以是0)

table:user_address

字段:id,user_id,类型(家庭,办公室)

用户可以拥有"家庭"地址(非强制性)和多个"办公室"地址.我有一个联接来获取用户地址,但我希望如果用户有一个"家"地址来获取该地址,而不是"办公室"地址.

那么,如果存在,我怎样才能获得"归属"地址,并且只有在不存在"办公室"地址时才能获得.(实际上,查询要复杂得多,并且连接在4-5个表上完成)

SELECT * FROM user LEFT JOIN user_address ON (user.id = address.user_id AND 
(user_address.type = "home" OR user.active_office_address_id = user_address.id))
group by user.id

Answer 1

您可以使用COALESCE()并加入地址表两次:

  SELECT user.id
       ,COALESCE(home.address, office.address) AS Address
  FROM user 
  LEFT JOIN user_address AS home
     ON user.id = home.user_id 
       AND home.type = "home"
  LEFT JOIN user_address AS office
     ON user.active_office_address_id = office.user_id 
  GROUP BY user.id

Answer 2

至少在SQL Server中，不确定MySql，可以case在order by子句中使用语句，例如：

order by user.id, case user_address.type when 'home' then 1 else 2 end, --additional ordering clauses here