三個表的MySQL查詢

Question

我有三個表與人，屬性和屬性的潛在價值。我找不出一個查詢來顯示所有的人，每個人的屬性和他們的缺失/空屬性。

下面是一個例子表...

attributes 
+---------------------+ 
| attribute_name (col)| 
+---------------------+ 
| name    | 
+---------------------+ 
| age     | 
+---------------------+ 
| gender    | 
+---------------------+ 
| email    | 
+---------------------+ 

people 
+-----------+----------+ 
| person_id | value_id | 
+-----------+----------+ 
| 2   | 7  | 
+-----------+----------+ 
| 2   | 9  | 
+-----------+----------+ 
| 3   | 8  | 
+-----------+----------+ 

values 
+---------------+----------------+-------+ 
| value_id (pk) | attribute_name | value | 
+---------------+----------------+-------+ 
| 7    | age   | 35 | 
+---------------+----------------+-------+ 
| 8    | age   | 28 | 
+---------------+----------------+-------+ 
| 9    | gender   | male | 
+---------------+----------------+-------+ 

如何加入三個表來顯示這樣的事情？

+-----------+----------+-----------------+--------+ 
| person_id | value_id | attribute_name | value | 
+-----------+----------+-----------------+--------+ 
| 2   | 7  | age    | 35  | 
+-----------+----------+-----------------+--------+ 
| 2   | 9  | gender   | male | 
+-----------+----------+-----------------+--------+ 
| 2   | NULL  | name   | NULL | 
+-----------+----------+-----------------+--------+ 
| 2   | NULL  | email   | NULL | 
+-----------+----------+-----------------+--------+ 
| 3   | 8  | age    | 28  | 
+-----------+----------+-----------------+--------+ 
| 3   | NULL  | gender   | NULL | 
+-----------+----------+-----------------+--------+ 
| 3   | NULL  | name   | NULL | 
+-----------+----------+-----------------+--------+ 
| 3   | NULL  | email   | NULL | 
+-----------+----------+-----------------+--------+ 

Answer 1

SELECT a.person_ID, 
     MAX(c.value_ID) value_ID, 
     b.attribute_name , 
     MAX(c.value) Value 
FROM people a 
     CROSS JOIN attributes b  
     LEFT JOIN `values` c 
      ON a.value_ID = c.value_ID AND 
       b.attribute_name = c.attribute_name 
GROUP BY a.person_ID, b.attribute_name 

• SQLFiddle Demo

Answer 2

我會用三個WHERE語句（SQL基礎）來連接這三個表。

您是否在發佈前做過搜索？

對於空存在這個帖子MySQL Show Null Results in Query - With INNER JOIN或本Mysql join four tables and show NULL value

;-)

Answer 3

select 
    person_id, 
    attributes.attribute_name as attribute_name, 
    values.value as value 
from people 
cross join attributes 
left join values on people.value_id=values.value_id 
       and attributes.attribute_name=values.attribute_name 

Answer 4

使用您的預計產量爲出發點，你本質上是想要的交叉產品貢品和不同的人people被加入people到values（n內部）連接的結果。這就是你如何在SQL中編寫腳本（特別是在MySQL中）：

SELECT 
    p.person_id, 
    v.value_id, 
    a.attribute_name, 
    v.value 
FROM 
    (SELECT DISTINCT person_id FROM people) p 
    CROSS JOIN attributes a 
    LEFT JOIN 
    people d 
    INNER JOIN `values` v ON d.value_id = v.value_id 
    ON p.person_id = d.person_id 
    AND a.attribute_name = v.attribute_name 
;