多次加入后，我得到了原始结果。
+----------------------------------------------------------------------+
| Results |
+----+----------+-------------+----------+-----------+-----------------+
| id | group_id | question_id | question | answer_id | answer | input |
+----+----------+-------------+----------+-----------+-----------------+
| 1 | 10001 | 1 | How old | 1 | 25 | NULL |
| 2 | 10001 | 2 | What like| 3 | Cola | NULL |
| 3 | 10001 | 2 | What like| 4 | Other | HotDog |
| 4 | 10001 | 3 | City | 5 | NYC | NULL |
| 5 | 10001 | 4 | Name | 7 | Other | Alex |
| 6 | 10002 | 1 | How old | 1 | 25 | NULL |
| 7 | 10002 | 2 | What like| 6 | Candy | NULL |
| 8 | 10002 | 3 | City | 8 | LA | NULL |
| 9 | 10002 | 4 | Name | 7 | Other | Roman |
+----+----------+-------------+----------+-----------+--------+--------+
但现在我想通过 group_id 在“单行 View ”中查看它。 如:
+----+----------+-------------+----------+-----------+
| id | How Old | What like | City | Name |
+----+----------+-------------+----------+-----------+
| 1 | 25 | Cola,HotDog | NYC | Alex |
| 2 | 25 | Candy | LA | Roman |
+----+----------+-------------+----------+-----------+
我不知道正常的 group_by/concat 构造。我必须做什么？
最佳答案
SET @i := 1;
SELECT @i := @i + 1 AS `id`
, GROUP_CONCAT(CASE WHEN question = 'How old' THEN answer ELSE NULL END) AS `How Old`
, GROUP_CONCAT(CASE WHEN question = 'What like' THEN IF(answer='Other', input, answer) ELSE NULL END) AS `What like`
, ....
FROM theTable
GROUP BY group_id
;
group_concat 忽略 null 值，仅当收到的唯一值为 null 时才返回 null。对于 CASE WHEN THEN ELSE END 语句，您可以轻松使用 IF(,,) ，就像在“其他”检查中使用的那样；但 CASE 更可移植(MS SQL Server 最近才添加了对此类 IF 的支持。)
关于mysql - 行到列的转换，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/36727725/