在MySQL中加入表的转置

如何解决在MySQL中加入表的转置

标题并不太合理，但这是我的表格结构

    users table
    +---------+
    | user_id |
    +---------+
    |       1 |
    |       2 |
    |       3 |
    +---------+

kids table

+----------+---------+------------+
| child_id | user_id | child_name |
+----------+---------+------------+
|        1 |       1 | child_a    |
|        2 |       1 | child_b    |
|        3 |       2 | child_c    |
|        4 |       3 | child_x    |
|        5 |       3 | child_y    |
|        6 |       3 | child_z    |
+----------+---------+------------+

现在，我想转置kids表并将其与users表联接。

预期输出。

+---------+---------+---------+---------+
| user_id | child_1 | child_2 | child_3 |
+---------+---------+---------+---------+
|       1 | child_a | child_b |         |
|       2 | child_c |         |         |
|       3 | child_x | child_y | child_z |
+---------+---------+---------+---------+

如您所见，由于用户的最大子代数为3，因此子信息栏共有3列。

到目前为止，我可以做的是以下操作，但是随后我将不得不接触其他一些语言来再次格式化此数据。

select user_id,group_concat(child_name) from kids group by user_id;

任何想法我都可以在mysql中获取预期结果吗？

编辑：我知道每个用户最多可以有3个孩子。

解决方法

@Shobi，我相信您可以通过将kids与users多次连接来做到这一点，这应该给您孩子的名字，例如：

SELECT `u`.`user_id`,`k`.`child_name` AS `child_1`,`k2`.`child_name` AS `child_2`,`k3`.`child_name` AS `child_3`
FROM `users` AS `u`
LEFT JOIN `kids` AS `k` ON `k`.`user_id` = `u`.`user_id`
LEFT JOIN `kids` AS `k2` ON `k2`.`user_id` = `u`.`user_id`
    AND `k2`.`child_id` <> `k`.`child_id`
LEFT JOIN `kids` AS `k3` ON `k3`.`user_id` = `u`.`user_id`
    AND `k3`.`child_id` <> `k2`.`child_id`
    AND `k3`.`child_id` <> `k`.`child_id`
GROUP BY `u`.`user_id`

Here is a working example on sqlfiddle.

注意：上面的查询在MySQL = 5.7中获得相同的结果，可以使用包含ANY_VALUE()的查询版本，该版本被列为适当的用法here in the MySQL documentation.

SELECT `u`.`user_id`,ANY_VALUE(`k`.`child_name`) AS `child_1`,ANY_VALUE(`k2`.`child_name`) AS `child_2`,ANY_VALUE(`k3`.`child_name`) AS `child_3`
FROM `users` AS `u`
LEFT JOIN `kids` AS `k` ON `k`.`user_id` = `u`.`user_id`
LEFT JOIN `kids` AS `k2` ON `k2`.`user_id` = `u`.`user_id`
    AND `k2`.`child_id` <> `k`.`child_id`
LEFT JOIN `kids` AS `k3` ON `k3`.`user_id` = `u`.`user_id`
    AND `k3`.`child_id` <> `k2`.`child_id`
    AND `k3`.`child_id` <> `k`.`child_id`
GROUP BY `u`.`user_id`
ORDER BY `u`.`user_id`

Here is a working example of this other version of the query in db-fiddle.

,

您的意思是透视表，不需要users表，可以尝试使用GROUP_CONCAT()这样的解析函数来动态执行以下方法，该方法包括ROW_NUMBER()函数： / p>

SET @sql = NULL;

SELECT GROUP_CONCAT(
             DISTINCT
             CONCAT(
                    'MAX(CASE WHEN rn =',rn,' THEN child_name END) AS child_',rn
                    )
       )
  INTO @sql
  FROM 
  (
   SELECT *,ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY child_id) AS rn
     FROM kids                    
  ) k;

SET @sql = CONCAT('SELECT user_id,',@sql,' FROM 
                     (
                      SELECT *,ROW_NUMBER() OVER 
                             (PARTITION BY user_id ORDER BY child_id) AS rn
                        FROM kids                    
                     ) k
                    GROUP BY user_id'); 

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

Demo