如何从两个表中以行作为列检索数据？ （MySQL中的动态数据透视）

Question

我的MySQL数据库中有两个表（表1和表2），就像下面的图像一样。 我想从表3中获取两个表中的数据，如果我的表1行增加，则表3列增加。 可以通过mysql通过一个查询获取数据吗？

CREATE TABLE `table_1` (
  `id` smallint(4) NOT NULL,
  `name` varchar(31) NOT NULL,
  `price` decimal(6,2) NOT NULL DEFAULT '0.00',
  `status` tinyint(1) NOT NULL DEFAULT '1'
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

INSERT INTO `table_1` (`id`, `name`, `price`, `status`) VALUES
(1, 'name1', '1.00', 1),
(2, 'name2', '2.00', 1),
(3, 'name3', '3.00', 1),
(4, 'name4', '5.00', 1),
(5, 'name5', '10.00', 1),
(6, 'name6', '15.00', 1),
(7, 'name7', '20.00', 1),
(8, 'name8', '50.00', 1);

CREATE TABLE `table_2` (
  `id` mediumint(6) NOT NULL,
  `table1_id` smallint(4) NOT NULL,
  `qry` smallint(4) NOT NULL DEFAULT '1',
  `total` decimal(6,2) NOT NULL DEFAULT '0.00',
  `date` date NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

INSERT INTO `table_2` (`id`, `table1_id`, `qry`, `total`, `date`) VALUES
(1, 1, 10, '10.00', '2017-12-02'),
(2, 2, 20, '40.00', '2017-12-02'),
(3, 3, 10, '30.00', '2017-12-02'),
(4, 5, 5, '50.00', '2017-12-02'),
(5, 4, 20, '100.00', '2017-12-03'),
(6, 6, 10, '150.00', '2017-12-03'),
(7, 7, 5, '100.00', '2017-12-03'),
(8, 8, 2, '100.00', '2017-12-03');

Answer 1

您的需求称为“动态数据透视查询”，因为在标准SQL中无法将数据转换为列标题。您必须编写一个将为您生成新查询的查询。

生成的查询代码将如下所示：

SELECT t2.`date`
 , max(CASE WHEN name = 'name1' THEN t2.total END) AS name1
 , max(CASE WHEN name = 'name2' THEN t2.total END) AS name2
 , max(CASE WHEN name = 'name3' THEN t2.total END) AS name3
 , max(CASE WHEN name = 'name4' THEN t2.total END) AS name4
 , max(CASE WHEN name = 'name5' THEN t2.total END) AS name5
 , max(CASE WHEN name = 'name6' THEN t2.total END) AS name6
 , max(CASE WHEN name = 'name7' THEN t2.total END) AS name7
 , max(CASE WHEN name = 'name8' THEN t2.total END) AS name8
FROM table_1 t1
INNER JOIN table_2 t2 ON t1.id = t2.id
GROUP BY t2.`date`

要创建该查询，请使用以下内容（这是动态SQL查询）

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'max(case when t1.name = ''',
      Name,
      ''' then t2.total end) AS ',
      replace(Name, ' ', '')
    )
  ) INTO @sql
from Table_1;

SET @sql = CONCAT(
'SELECT t2.`date`, '
, @sql
, ' from table_1 t1
inner join table_2 t2 on t1.id = t2.id
group by t2.`date`');

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

请参阅this Demo

+---------------------+-------+-------+-------+-------+--------+--------+--------+--------+
|        date         | name1 | name2 | name3 | name4 | name5  | name6  | name7  | name8  |
+---------------------+-------+-------+-------+-------+--------+--------+--------+--------+
| 02.12.2017 00:00:00 | 10,00 | 40,00 | 30,00 | 50,00 | NULL   | NULL   | NULL   | NULL   |
| 03.12.2017 00:00:00 | NULL  | NULL  | NULL  | NULL  | 100,00 | 150,00 | 100,00 | 100,00 |
+---------------------+-------+-------+-------+-------+--------+--------+--------+--------+