在mysql中选择动态列

是否可以像这样遍历表：

mysql> select * from `stackoverflow`.`Results`; +--------------+---------+-------------+--------+ | ID | TYPE | CRITERIA_ID | RESULT | +--------------+---------+-------------+--------+ | 1 | car | env | 1 | | 2 | car | gas | | | 3 | car | age | | | 4 | bike | env | 1 | | 5 | bike | gas | | | 6 | bike | age | 1 | | 7 | bus | env | 1 | | 8 | bus | gas | 1 | | 9 | bus | age | 1 | +--------------+---------+-------------+--------+ 9 rows in set (0.00 sec)

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'SUM(IF(CRITERIA_ID = ''',
      CRITERIA_ID,
      ''', RESULT, NULL)) AS ',
      CRITERIA_ID
    )
  ) INTO @sql
FROM
  Results;
SET @sql = CONCAT('SELECT type, ', @sql, ' FROM Results GROUP BY type');

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

---

不幸的是，MySQL没有一个

PIVOT

函数，这基本上就是您想要做的。因此，您需要将聚合函数与

CASE

语句一起使用：

SELECT type,
  sum(case when criteria_id = 'env' then result end) env,
  sum(case when criteria_id = 'gas' then result end) gas,
  sum(case when criteria_id = 'age' then result end) age
FROM results
group by type

看

现在，如果您希望动态执行此操作，这意味着您无法提前知道要转置的列，那么您应该阅读以下文章：

您的代码如下所示：

mysql> select * from `stackoverflow`.`Results`; +--------------+---------+-------------+--------+ | ID | TYPE | CRITERIA_ID | RESULT | +--------------+---------+-------------+--------+ | 1 | car | env | 1 | | 2 | car | gas | | | 3 | car | age | | | 4 | bike | env | 1 | | 5 | bike | gas | | | 6 | bike | age | 1 | | 7 | bus | env | 1 | | 8 | bus | gas | 1 | | 9 | bus | age | 1 | +--------------+---------+-------------+--------+ 9 rows in set (0.00 sec)

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'SUM(IF(CRITERIA_ID = ''',
      CRITERIA_ID,
      ''', RESULT, NULL)) AS ',
      CRITERIA_ID
    )
  ) INTO @sql
FROM
  Results;
SET @sql = CONCAT('SELECT type, ', @sql, ' FROM Results GROUP BY type');

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

---

参见

+1，了解准备好的报表的实施情况。动态SQL可能被低估了。警告：不能在函数或触发器中使用。的可能重复