问题描述
MySQL 是否可以将列转换为行，动态添加行所需的列数.我认为我的问题可能与数据透视表有关，但我不确定，除了给出以下示例之外，我不知道如何构建这个问题.
Can MySQL convert columns into rows, dynamically adding as many columns as are needed for the rows. I think my question might be related to pivot tables but I'm unsure and I don't know how to frame this question other than by giving the following example.
给定两个表 A 和 B，它们看起来像
Given a two tables A and B, which look like
表A
+--+-----+----+
|id|order|data|
+--+-----+----+
|1 |1 |P |
+--+-----+----+
|2 |2 |Q |
+--+-----+----+
|2 |1 |R |
+--+-----+----+
|1 |2 |S |
+--+-----+----+
我喜欢编写如下所示的查询:
I like to write a query that looks like the following:
结果表
+--+-----+-----+
|id|data1|data2|
+--+-----+-----+
|1 |P |S |
+--+-----+-----+
|2 |R |Q |
+--+-----+-----+
基本上我想把表 B 中的每一行变成结果表中的一列.如果 id=1 的表 B 中添加了一个新条目，那么我希望结果表自动扩展一列以容纳这个额外的数据点.
Basically I want to turn each row in table B into a column in the result table. If there was a new entry was added to table B for id=1, then I want the result table to automatically extend by one column to accommodate this extra data point.
推荐答案
您可以使用 GROUP BY 和 MAX 来模拟枢轴.MySQL 也支持 IF 语句.
You can use GROUP BY and MAX to simulate pivot. MySQL also supports IF statement.
SELECT ID,
MAX(IF(`order` = 1, data, NULL)) data1,
MAX(IF(`order` = 2, data, NULL)) data2
FROM TableA
GROUP BY ID
- SQLFiddle 演示
如果order有多个值，动态SQL可能更合适，这样你就不必修改查询:
If you have multiple values of order, dynamic SQL may be more appropriate so that you will not have to modify the query:
SET @sql = NULL;
SELECT
GROUP_CONCAT(DISTINCT
CONCAT(
'MAX(IF(`order` = ', `order`, ',data,NULL)) AS data', `order`)
) INTO @sql
FROM TableName;
SET @sql = CONCAT('SELECT ID, ', @sql, '
FROM TableName
GROUP BY ID');
PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
- SQLFiddle 演示
- SQLFiddle 演示(另一个例子)
两个查询的输出:
╔════╦═══════╦═══════╗
║ ID ║ DATA1 ║ DATA2 ║
╠════╬═══════╬═══════╣
║ 1 ║ P ║ S ║
║ 2 ║ R ║ Q ║
╚════╩═══════╩═══════╝
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