MySQL根据主数据表查询只有key和value的数据项表

Question

我有以下表格

data [table]
- id [primary key]
- context

data_items [table]
- item_id [primary key]
- data_id [foreign key]
- key
- value

对于“数据”表，它将包含类似的内容

id    context
--    -------
 1    car
 2    office
 3    car
 4    food

对于“data_items”表，它将包含父“data”表中的项目，例如

item_id   data_id   key     value
-------   -------   -----   ------------
      1         1   name    Benz
      2         1   model   C-Class 
      3         1   price   1000000

      4         2   city    New York
      5         2   name    Empire State
      6         2   floor   5th
      7         2   room    504

      8         1   name    BMW
      9         1   model   528i Sedan
     10         1   price   80000

     11         3   name    Ford
     12         3   model   Hybrid SE
     13         3   price   30000

     14         3   name    Ford
     15         3   model   528i Sedan
     16         3   price   80000

     17         3   name    BMW
     18         3   model   528i Sedan
     19         3   price   80000

     20         2   city    New York
     21         2   name    Chrystler
     22         2   floor   2nd
     23         2   room    24A

     24         4   type    American
     25         4   name    Hamburger
     26         4   price   $50

     27         4   type    Chinese
     28         4   name    Noodle
     29         4   price   $20

     30         2   city    Los Angeles
     31         2   name    US Bank Tower
     32         2   floor   10th
     33         2   room    105C

所以如果我运行类似的 SQL 查询

SELECT * FROM data_items ........ WHERE data_id = 2;

它应该给我以下列表

data_id   city          Name             Floor   room
-------   --------      -------------    -----   ----
      2   New York      Empire State     5th     504
      2   New York      Chrysler         2nd     24A
      2   Los Angeles   US Bank Tower    10th    105C

如果我运行以下 SQL 查询

SELECT * FROM data_items ........ WHERE data_id = 4;

它应该给我以下列表

data_id   type          Name             price
-------   --------      -------------    ----
      4   American      Hambergur        $50
      4   Chinese       Noodle           $20

所以我的问题是:

1. 我需要第三个表来连接这两个表吗？
2. 获取上述结果的确切 SQL 命令是什么？

谢谢。

Answer 1

这种类型的表称为 EAV model (实体-属性-值模型)，尽管它们在插入数据时提供了很大的自由度，但该数据比规范化模型更难查询(但在某些情况下它们很有用)。对于您的特定问题，您需要事先知道要查询的 data_id 的值是多少，并对每个 data_id 使用不同的查询。例如，要获取 data_id = 2 的结果:

SELECT  `data_id`,
        MIN(CASE WHEN `key` = 'city' THEN `value` END) AS `city`,
        MIN(CASE WHEN `key` = 'name' THEN `value` END) AS `name`,
        MIN(CASE WHEN `key` = 'floor' THEN `value` END) AS `floor`,
        MIN(CASE WHEN `key` = 'room' THEN `value` END) AS `room`
FROM data_items
WHERE data_id = 2
GROUP BY `data_id`

对于 data_id = 4:

SELECT  `data_id`,
        MIN(CASE WHEN `key` = 'type' THEN `value` END) AS `type`,
        MIN(CASE WHEN `key` = 'name' THEN `value` END) AS `name`,
        MIN(CASE WHEN `key` = 'price' THEN `value` END) AS `price`
FROM data_items
WHERE data_id = 4
GROUP BY `data_id`