SQL语句 - SQL Matrix

Question

对于下表，是否可以创建SQL语句来创建数据矩阵或视图？

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 

结果矩阵：

 | 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

有两种方法，你可以在枢轴数据在MySQL中。如果您事先知道这些值（团队），那么您将对这些值进行硬编码，或者您可以使用准备好的语句来生成动态SQL。

静态版本是：

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; 

见SQL Fiddle with Demo

如果你想使用一个事先准备好的声明动态版，代码如下：

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; 

见SQL Fiddle with Demo 。

编辑＃1，在想到这个后，我实际上会做这个稍微有点不同。我会在队伍出现在行和列中的数据中生成一个真正的矩阵。要做到这一点，你会首先使用UNION ALL查询来获取所有的球队在两列：

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

见SQL Fiddle with Demo。一旦做到这一点，那么你会支点数据：

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; 

见SQL Fiddle with Demo。它给出了更详细的结果：

| 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

您不能创建具有可变数量的列的SQL语句或视图。在标准SQL中，可以通过执行类似以下操作来转发数据：

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 

某些数据库支持一个pivot语句。

一般来说，您必须创建一个SQL语句作为字符串，然后执行它（通常称为动态 SQL）。这样的语句可以由SQL，存储过程，Excel或其他编程工具生成。然后它需要被执行。

让我重复一遍：任何给定的SQL语句都有一组预定义的列。你不能改变列的数量。