我相信这种性质的所有问题都是这样开始的......
我有两个表：fields和data。 Fields描述了（不存在的）表的列名，data包含该（不存在的）表的数据。像这样......
字段：
+----+---------+-------------+---------------+-------+----------+
| ID | F_ORDER | NAME | LABEL | VALUE | TYPE |
+----+---------+-------------+---------------+-------+----------+
| 1 | 1 | IS_EMPLOYEE | Region | | checkbox |
| 2 | 3 | EM_AVATAR | Avatar | | avatar |
| 3 | 4 | EM_JOBTITLE | Job Title | | text |
| 4 | 5 | EM_COMPANY | Company | | text |
| 5 | 6 | EM_PHONE | Phone | | text |
| 6 | 2 | EM_ORDER | Display Order | 5 | text |
+----+---------+-------------+---------------+-------+----------+
数据：
+-----+----------+---------+--------------------------------------+
| ID | FIELD_ID | USER_ID | VALUE |
+-----+----------+---------+--------------------------------------+
| 5 | 1 | 1 | YES |
| 6 | 2 | 1 | |
| 7 | 3 | 1 | Owner |
| 8 | 4 | 1 | Acme, Inc. |
| 9 | 5 | 1 | 123-456-7987 |
| 150 | 5 | 31 | 123-623-5555 |
| 149 | 4 | 31 | Acme, Inc. |
| 148 | 3 | 31 | Sales and Customer Support |
| 147 | 2 | 31 | |
| 146 | 1 | 31 | YES |
| 26 | 1 | 6 | NO |
| 27 | 2 | 6 | http://example.com/avi/avi.jpeg |
| 28 | 3 | 6 | CEO |
| 29 | 4 | 6 | Acme |
| 30 | 5 | 6 | (123) 734-5555 |
| 31 | 1 | 7 | NO |
| 32 | 2 | 7 | http://example.com/avi/avi.jpeg |
| 33 | 3 | 7 | VP, Services |
| 34 | 4 | 7 | Acme |
| 35 | 5 | 7 | (913) 963-5555 |
| 36 | 1 | 14 | NO |
| 37 | 2 | 14 | http://example.com/avi/avi.jpeg |
| 38 | 3 | 14 | Senior Accountant |
| 39 | 4 | 14 | Acme |
| 40 | 5 | 14 | (123) 213-5555 |
| 41 | 1 | 10 | NO |
| 42 | 2 | 10 | http://example.com/avi/avi.jpeg |
| 43 | 3 | 10 | President |
| 44 | 4 | 10 | Acme |
| 45 | 5 | 10 | (123) 734-5555 |
| 46 | 1 | 12 | NO |
| 47 | 2 | 12 | http://example.com/avi/avi.jpeg |
| 48 | 3 | 12 | Services Supervisor |
| 49 | 4 | 12 | Acme |
| 50 | 5 | 12 | (123) 573-5555 |
| 51 | 1 | 11 | NO |
| 52 | 2 | 11 | http://example.com/avi/avi.jpeg |
| 53 | 3 | 11 | Operations Supervisor |
| 54 | 4 | 11 | Acme |
| 55 | 5 | 11 | (123) 259-5555 |
| 56 | 1 | 8 | NO |
| 57 | 2 | 8 | http://example.com/avi/avi.jpeg |
| 58 | 3 | 8 | General Information |
| 59 | 4 | 8 | Acme |
| 60 | 5 | 8 | (123) 213-5555 |
| 61 | 1 | 9 | NO |
| 62 | 2 | 9 | http://example.com/avi/avi.jpeg |
| 63 | 3 | 9 | VP, Sales |
| 64 | 4 | 9 | Acme |
| 65 | 5 | 9 | (123) 210-5555 |
+-----+----------+---------+--------------------------------------+
我正在寻找的查询的基本措辞是：我希望所有员工的所有信息（IS_EMPLOYEE = "YES"）并按其显示顺序列（EM_ORDER）排序。
到目前为止，我的查询无处可寻。我得到的结果有点像这样：
+--------+------------+------------+-------+---------+-------+
ID FIELD_ID NAME LABEL TYPE VALUE
+--------+------------+------------+-------+---------+-------+
7 1 IS_EMPLOYEE Region checkbox YES
+--------+------------+------------+-------+---------+-------+
我需要的是这样的结果：
+-------+------------+---------+-----------+----------+-------------+--------+
USER_ID IS_EMPLOYEE EM_AVATAR EM_JOBTITLE EM_COMPANY EM_PHONE EM_ORDER
+-------+------------+---------+-----------+----------+-------------+--------+
6 YES http:// CEO Acme 123-123-555 5
+-------+------------+---------+-----------+----------+-------------+--------+
当然，我正试图将它作为一个可用的数组（$results['user_id']['jobtitle']等）重新用于PHP。我可以得到一切并使用PHP完成它，但我正在尝试学习MySQL，我认为这是一个比做几个foreach块更快的方法...虽然我可能是错的。
提前感谢您的帮助。
答案 0 :(得分：2)
您的数据表以Entity-Attribute-Value方式组织，这不是存储关系数据的有效方式。毫无疑问，使用SQL将其检索到每个属性只有一列的传统行中是很尴尬的。
要使用SQL执行此操作，您基本上需要执行数据透视查询。这很难写，而且执行效率非常低。
最好为员工提取所有数据：
SELECT e.*, f.label
FROM Data AS is_employee
JOIN Data AS e USING (user_id)
JOIN Fields AS f ON e.field_id = f.id
WHERE (is_employee.field_id, is_employee.value) = (1, 'Yes');
然后将其重组为PHP代码中的关联数组集合：
while ($row = $stmt->fetch()) {
$data[ $row['user_id'] ][ $row['label'] ] = $row['value'];
}
答案 1 :(得分：1)
这基本上是PIVOT，但MySQL没有PIVOT函数，所以你可以使用聚合函数和CASE语句来复制它。
静态版本是指您想知道要转换为列的所有值（这些是您的字段名称）：
select
d.user_id,
max(case when f.name = 'IS_EMPLOYEE' then d.value else null end) IS_EMPLOYEE,
max(case when f.name = 'EM_AVATAR' then d.value else null end) EM_AVATAR,
max(case when f.name = 'EM_JOBTITLE' then d.value else null end) EM_JOBTITLE,
max(case when f.name = 'EM_COMPANY' then d.value else null end) EM_COMPANY,
max(case when f.name = 'EM_PHONE' then d.value else null end) EM_PHONE,
max(case when f.name = 'EM_ORDER' then d.value else null end) EM_ORDER
from data d
left join fields f
on f.id = d.FIELD_ID
group by d.user_id
结果：
| USER_ID | IS_EMPLOYEE | EM_AVATAR | EM_JOBTITLE | EM_COMPANY | EM_PHONE | EM_ORDER |
---------------------------------------------------------------------------------------------------------------------------------
| 1 | YES | (null) | Owner | Acme, Inc. | 123-456-7987 | (null) |
| 6 | NO | http://example.com/avi/avi.jpeg | CEO | Acme | (123) 734-5555 | (null) |
| 7 | NO | http://example.com/avi/avi.jpeg | VP, Services | Acme | (913) 963-5555 | (null) |
| 8 | NO | http://example.com/avi/avi.jpeg | General Information | Acme | (123) 213-5555 | (null) |
| 9 | NO | http://example.com/avi/avi.jpeg | VP, Sales | Acme | (123) 210-5555 | (null) |
| 10 | NO | http://example.com/avi/avi.jpeg | President | Acme | (123) 734-5555 | (null) |
| 11 | NO | http://example.com/avi/avi.jpeg | Operations Supervisor | Acme | (123) 259-5555 | (null) |
| 12 | NO | http://example.com/avi/avi.jpeg | Services Supervisor | Acme | (123) 573-5555 | (null) |
| 14 | NO | http://example.com/avi/avi.jpeg | Senior Accountant | Acme | (123) 213-5555 | (null) |
| 31 | YES | (null) | Sales and Customer Support | Acme, Inc. | 123-623-5555 | (null) |
如果您将未知数量的值转换为列，则可以使用预准备语句动态生成sql。
SET @sql = NULL;
SELECT
GROUP_CONCAT(DISTINCT
CONCAT(
'max(case when f.name = ''',
name,
''' then d.value end) AS ',
name
)
) INTO @sql
FROM fields;
SET @sql = CONCAT('SELECT d.user_id, ', @sql, '
from data d
left join fields f
on f.id = d.FIELD_ID
GROUP BY d.user_id');
PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
两者都会产生相同的结果。
然后，您可以添加WHERE子句以过滤掉任何不需要的行。