可以替代GROUP_CONCAT嗎？ 多個聯接到同一個表，不同的列

Question

我什至在這里不確定正確的術語。 MySQL新手，或多或少。

給定幾個表，定義如下：

    CREATE TABLE users 
 ( user_id int(11) NOT NULL auto_increment
 , name VARCHAR(255)
 , pri_location_id mediumint(8)
 , sec_location_id mediumint(8)
 , PRIMARY KEY  (user_id)
 );

 CREATE TABLE locations 
 ( location_id mediumint(8) NOT NULL AUTO_INCREMENT
 , name varchar(255)
 , PRIMARY KEY (location_id)
 )

我正在嘗試進行查詢以一次性獲取用戶名以及主要和次要位置。

我可以得到這樣的一個：

SELECT u.name AS user_name, l.name as primary_location FROM users u, locations l WHERE u.primary_location_id=l.location_id

但是我在正確的語法上繪制了一個空白，用來在一個查詢中同時獲取兩者。

Answer 1

SELECT u.name AS user_name, l1.name as primary_location , l2.name as secondary_location
FROM users u
JOIN locations l1 ON(u.pri_location_id=l1.location_id)
JOIN locations l2 ON(u.sec_location_id = l2.location_id);

Answer 2

首先，如果可以的話，我會強烈考慮更改您的數據庫架構，以添加一個users_locations表，該表可用於正確描述這種多對多關系。

該表如下所示：

user_id    location_id    location_type
1          1              primary
1          2              secondary
2          1              secondary
2          2              primary

等等。

您可能希望在所有三列中使用復合主鍵。 而location_type最好是枚舉數據類型。

您的查詢將像

SELECT
    u.name AS user_name
    l.name AS location_name
    ul.location_type AS location_type
FROM users AS u
INNER JOIN user_location AS ul /* possibly use left join here if user can exist without a location */
  ON u.user_id = ul.user_id
INNER JOIN locations AS l
  ON ul.location_id = l.location_id
ORDER BY ul.location_type ASC

每個用戶最多返回兩個記錄（主要和次要的記錄分開，主要記錄在前）

如果您需要將此折疊成一條記錄，則可以執行以下操作：

SELECT
    u.name AS user_name
    COALESCE(CASE WHEN ul.location_type = 'primary' THEN l.name ELSE NULL END CASE) AS primary_location,
    COALESCE(CASE WHEN ul.location_type = 'secondary' THEN l.name ELSE NULL END CASE) AS secondary_location
FROM users AS u
INNER JOIN user_location AS ul /* possibly use left join here if user can exist without a location */
  ON u.user_id = ul.user_id
INNER JOIN locations AS l
  ON ul.location_id = l.location_id
GROUP BY `user_name`

但是，如果您陷於當前架構，那么@Jlil的解決方案應該為您工作。