使用CREATE VIEW将列值拆分为多个列

Question

使用MySQL查询，如何像示例A那样获取表：

Example A
+------+---------------+----------+
| id   | value         | class    |
+------+---------------+----------+
| 1    | 33.00         | total    |
| 1    | 12.00         | shipping |
| 2    | 45.00         | total    |
| 2    | 15.00         | shipping |
+------+---------------+----------+

并创建一个类似于示例B的视图？

Example B
+------+---------------+---------------+
| id   | value_total   | value_shipping|
+------+---------------+---------------+
| 1    | 33.00         | 12.00         |
| 2    | 45.00         | 15.00         |
+------+---------------+---------------+

Answer 1

您可以简单地使用SUM()函数：

SELECT id
,SUM(CASE WHEN class = 'total' THEN value ELSE 0 END) AS value_total
,SUM(CASE WHEN class = 'shipping' THEN value ELSE 0 END) AS value_shipping
FROM Table1 
GROUP BY id;

看到这个SQLFiddle

如果您的 班级 数目未知，请尝试此动态查询

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'SUM(case when class = ''',
      class,
      ''' then value else 0 end) AS `value_',
      class, '`'
    )
  ) INTO @sql
FROM Table1;


SET @sql = CONCAT('SELECT id, ', @sql, '
                  FROM Table1 
                  GROUP BY id');

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

输出：

╔════╦═════════════╦════════════════╗
║ ID ║ VALUE_TOTAL ║ VALUE_SHIPPING ║
╠════╬═════════════╬════════════════╣
║  1 ║          33 ║             12 ║
║  2 ║          45 ║             15 ║
╚════╩═════════════╩════════════════╝

看到这个SQLFiddle