MYSQL子查詢行作爲主查詢中的列

Question

在我們的數據庫中，我們有一個用戶表和其他數據的鍵值表。我們一直試圖提出一個查詢來加入這兩個，將k-v表中的鍵作爲列標題，將值作爲字段。

我們作爲目前唯一的解決辦法是GROUP_CONCAT鍵 - 值對每個用戶爲一列，然後分析它們的查詢輸出緩慢和壞後...

這裏是一般設置：

的user.db表：

------------------------------ 
| uid | firstname | lastname | 
------------------------------ 
| 01 | john  | doe  | 
| 02 | jane  | doe  | 
------------------------------ 

----------------------------- 
| uid | question | answer | 
----------------------------- 
| 01 | question1 | answer1 | 
| 01 | question2 | answer2 | 
| 02 | question1 | answer3 | 
| 02 | question2 | answer4 | 
----------------------------- 

我們想查詢結果來獲得：

------------------------------------------------------ 
| uid | firstname | lastname | question1 | question2 | 
------------------------------------------------------ 
| 01 | john  | doe  | answer1 | answer2 | 
| 02 | jane  | doe  | answer3 | answer4 | 
------------------------------------------------------ 

我希望有一個簡單的方法來做到這一點，但一直沒能找到任何東西。所有的幫助將不勝感激。

Answer 1

在其他數據庫的，你可以使用PIVOT功能，但MySQL不具有功能，因此將使用聚合功能和CASE語句來複制它。如果你知道所有的值，你可以硬編碼類似這樣的價值觀：

select u.uid, u.firstname, u.lastname, 
    max(case when question='question1' then answer else null end) as question1, 
    max(case when question='question2' then answer else null end) as question2 
from users u 
left join kv 
    on u.uid = kv.uid 
group by u.uid, u.firstname, u.lastname; 

見SQL Fiddle with demo

但如果你有未知的值，那麼你可以使用一個prepared statement生成動態SQL：

SET @sql = NULL; 
SELECT 
    GROUP_CONCAT(DISTINCT 
    CONCAT(
     'max(case when question = ''', 
     question, 
     ''' then answer else NULL end) AS ', 
     question 
    ) 
) INTO @sql 
FROM kv; 

SET @sql = CONCAT('SELECT u.uid, u.firstname, u.lastname, ', @sql, ' 
        from users u 
        left join kv 
        on u.uid = kv.uid 
        group by u.uid, u.firstname, u.lastname'); 

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

見SQL Fiddle with Demo

兩個版本都將產生相同的結果：

| UID | FIRSTNAME | LASTNAME | QUESTION1 | QUESTION2 | 
------------------------------------------------------ 
| 1 |  john |  doe | answer1 | answer2 | 
| 2 |  jane |  doe | answer3 | answer4 | 

準備語句的好處是，如果您有更改值，那麼這將在運行時生成列的列表。

Answer 2

這將模擬數據透視表：

Select 
    uid, 
    firstname, 
    lastname, 
    max(case when question = 'question1' then answer end) as question1, 
    max(case when question = 'question2' then answer end) as question2 
From 
    users inner join answers on users.uid = answers.uid 
Grop by uid, firstname, lastname 

還有一個解決方案與聯接：

Select uid, firstname, lastname, 
    answers_1.answer as question1, 
    answers_2.answer as question2 
From 
    users left join answers answers_1 
    on users.uid = answers_1.uid and answers_1.question = 'question1' 
    left join answers answers_2 
    on users.uid = answers_2.uid and answers_2.question = 'question2' 

當然，你必須提前知道什麼問題是。如果情況並非如此，據我所知，由於MySql不支持PIVOT，因此無法使用標準SQL來回答您的問題。

Answer 3

你可以嘗試這樣的事情：

select u.uid, u.firstname, u.lastname, 
    max(case when question="question1" then answer else null) as question1, 
    max(case when question="question2" then answer else null) as question2 
from user u join answers a on u.uid = a.uid 
group by u.uid, u.firstname, u.lastname