SQL語句 - SQL Matrix

Question

對於下表，是否可以創建SQL語句來創建數據矩陣或視圖？

表：

TeamA|TeamB|Won|Lost 
-------------------- 
    A | B | 5 | 3 
    A | C | 2 | 4 
    A | D | 9 | 1 
    B | E | 5 | 5 
    C | A | 2 | 4 

結果矩陣：

 | A | B | C | D | E 
---------------------------- 
    A | 0 | 2 | -2 | 8 | 0 
    B | 0 | 0 | 0 | 0 | 0 
    C | -2 | 0 | 0 | 0 | 0 

Answer 1

有兩種方法，你可以在樞軸數據在MySQL中。如果您事先知道這些值（團隊），那麼您將對這些值進行硬編碼，或者您可以使用準備好的語句來生成動態SQL。

靜態版本是：

select TeamA, 
    max(case when TeamB = 'A' then won - lost else 0 end) as A, 
    max(case when TeamB = 'B' then won - lost else 0 end) as B, 
    max(case when TeamB = 'C' then won - lost else 0 end) as C, 
    max(case when TeamB = 'D' then won - lost else 0 end) as D, 
    max(case when TeamB = 'E' then won - lost else 0 end) as E 
from yourtable 
group by TeamA; 

見SQL Fiddle with Demo

如果你想使用一個事先準備好的聲明動態版，代碼如下：

SET @sql = NULL; 
SELECT 
    GROUP_CONCAT(DISTINCT 
    CONCAT(
     'MAX(CASE WHEN TeamB = ''', 
     TeamB, 
     ''' THEN won - lost else 0 END) AS `', 
     TeamB, '`' 
    ) 
) INTO @sql 
from 
(
    select * 
    from yourtable 
    order by teamb 
) x; 

SET @sql 
    = CONCAT('SELECT TeamA, ', @sql, ' 
      from yourtable 
      group by TeamA'); 

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

見SQL Fiddle with Demo 。

編輯＃1，在想到這個後，我實際上會做這個稍微有點不同。我會在隊伍出現在行和列中的數據中生成一個真正的矩陣。要做到這一點，你會首先使用UNION ALL查詢來獲取所有的球隊在兩列：

select teama Team1, teamb Team2, 
    won-lost Total 
from yourtable 
union all 
select teamb, teama, 
    won-lost 
from yourtable 

見SQL Fiddle with Demo。一旦做到這一點，那麼你會支點數據：

select Team1, 
    coalesce(max(case when Team2 = 'A' then Total end), 0) as A, 
    coalesce(max(case when Team2 = 'B' then Total end), 0) as B, 
    coalesce(max(case when Team2 = 'C' then Total end), 0) as C, 
    coalesce(max(case when Team2 = 'D' then Total end), 0) as D, 
    coalesce(max(case when Team2 = 'E' then Total end), 0) as E 
from 
(
    select teama Team1, teamb Team2, 
    won-lost Total 
    from yourtable 
    union all 
    select teamb, teama, 
    won-lost 
    from yourtable 
) src 
group by Team1; 

見SQL Fiddle with Demo。它給出了更詳細的結果：

| TEAM1 | A | B | C | D | E | 
------------------------------- 
|  A | 0 | 2 | -2 | 8 | 0 | 
|  B | 2 | 0 | 0 | 0 | 0 | 
|  C | -2 | 0 | 0 | 0 | 0 | 
|  D | 8 | 0 | 0 | 0 | 0 | 
|  E | 0 | 0 | 0 | 0 | 0 | 

Answer 2

您不能創建具有可變數量的列的SQL語句或視圖。在標準SQL中，可以通過執行類似以下操作來轉發數據：

select TeamA, 
     max(case when TeamB = 'A' then won - lost end) as A, 
     max(case when TeamB = 'B' then won - lost end) as B, 
     max(case when TeamB = 'C' then won - lost end) as C, 
     max(case when TeamB = 'D' then won - lost end) as D, 
     max(case when TeamB = 'E' then won - lost end) as E 
from t 
group by TeamA 
order by 1 

某些數據庫支持一個pivot語句。

一般來說，您必須創建一個SQL語句作爲字符串，然後執行它（通常稱爲動態 SQL）。這樣的語句可以由SQL，存儲過程，Excel或其他編程工具生成。然後它需要被執行。

讓我重複一遍：任何給定的SQL語句都有一組預定義的列。你不能改變列的數量。