Question

我有2个表格，其结构与此类似：

table：user

字段：id，active_office_address_id（可以是0）

table：user_address

字段：id，user_id，type（home，office）

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，但是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

两个左连接和一个case语句将为您提供所需的地址ID。

SELECT user.*,CASE WHEN home_addr.id IS NOT NULL THEN home_addr.id ELSE ofc_addr.id END AS addr_id
FROM user
LEFT JOIN user_address AS home_addr
  ON (user.id = home_addr.user_id AND home_addr.type = 'home')
LEFT JOIN user_address AS ofc_addr
  ON (user.active_office_address_id = ofc_addr.id)

您可以将此作为特定用户的子选项反馈：

SELECT * FROM user LEFT JOIN user_address
WHERE user.id = ?
AND user_address.user_id = user.id
AND user_address.id IN
    (SELECT CASE WHEN home_addr.id IS NOT NULL THEN home_addr.id ELSE ofc_addr.id END AS addr_id
    FROM user
    LEFT JOIN user_address AS home_addr
      ON (user.id = home_addr.user_id AND home_addr.type = 'home')
    LEFT JOIN user_address AS ofc_addr
      ON (user.active_office_address_id = ofc_addr.id)
    WHERE user.id = ?)

这假设每个用户只存在一个家庭地址。

Answer 3

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

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

SQL - LEFT JOIN多个条件 - 优先级

3 个答案: