Question

每个人我想用动态行执行MySQL查询。

想象一下：

table phones_categories

+----+------------+
| id | name       |
+----+------------+
|  1 | Home Phone |
|  2 | Cell Phone | 
|  3 | Fax        | 
+----+------------+

桌面电话

+----+-----------+-------------------+--------------+
| id | entity_id | phone_category_id | phone_number |
+----+-----------+-------------------+--------------+
|  1 |         1 |                 1 | X19 XXX 2XX  |
|  2 |         1 |                 3 | X19 XXX 2XX  |
|  3 |         2 |                 1 | X18 XXX 4XX  |
|  4 |         2 |                 3 | X18 XXX 4XX  |
+----+-----------+-------------------+--------------+

我想得到以下输出：

+-----------+--------------+--------------+-------------+
| entity_id | Home Phone   | Cell Phone   | Fax         |
+-----------+--------------+--------------+-------------+
|         1 |  X19 XXX 2XX |              | X19 XXX 2XX |             
|         2 |  X18 XXX 4XX |              | X18 XXX 4XX |
+-----------+--------------+--------------+-------------+

好的，我需要一些“动态”因为未来表phone_categories可以增长。

Answer 1

这称为“数据透视表”或“交叉表查询”。仅MySQL不能动态地执行此操作。您总是需要提前知道列名，因此如果您使用编程/脚本语言输出，则可以在确定后使用它以for循环动态构建SQL语句。类别。

但查询将如下所示：

SELECT
  phones.entity_id,
  CASE WHEN phones.phone_category_id = 1 THEN phones.phone_number ELSE NULL END AS `Home Phone`,
  CASE WHEN phones.phone_category_id = 2 THEN phones.phone_number ELSE NULL END AS `Cell Phone`,
  CASE WHEN phones.phone_category_id = 3 THEN phones.phone_number ELSE NULL END AS `Fax`
FROM phones

你还没有发现任何编程语言，所以这里有一些伪代码来生成查询：

categories = "SELECT id, name FROM phone_categories;"
foreach categories
  sql_columns = sql_columns + " CASE WHEN phones.phone_category_id = " + categories.id + " THEN phones.phone_number ELSE NULL END AS  `categories.name`

mysql动态行

1 个答案: