MySql的多列从单列表选择与分组

Question

我想要显示像这样的数据我在哪里GROUP上DATE：

Date  Star1 Star2 Star3 Star4 Star5 
2012-10-25 null null null  1  1 

我目前的表有三（3）列，显示：

ID DATE  RATING 
5 2012-10-25 4 
6 2012-10-25 5 

我使用此查询来尝试实现上述数据：

SELECT 
    DATE, 
    CASE 
     WHEN RATING = 1 THEN 1 
     ELSE NULL 
    END AS STAR1, 
    CASE 
     WHEN RATING = 2 THEN 1 
     ELSE NULL 
    END AS STAR2, 
    CASE 
     WHEN RATING = 3 THEN 1 
     ELSE NULL 
    END AS STAR3, 
    CASE 
     WHEN RATING = 4 THEN 1 
     ELSE NULL 
    END AS STAR4, 
    CASE 
     WHEN RATING = 5 THEN 1 
     ELSE NULL 
    END AS STAR5 
FROM 
    table1 
WHERE 
    DATE = '2012-10-25' 
GROUP BY DATE 
ORDER BY DATE 

这只能吐出一个等级寿：

Date  Star1 Star2 Star3 Star4 Star5 
2012-10-25 null null null null  1 

它像它不是在日期分组，我怎么能做到这一点？

Answer 1

MySQL允许这样的查询有点令人惊讶，因为GROUP BY并未涵盖SELECT列表中的所有非聚合列。

如果你换你的CASE语句为计数，那么我想你会与你想要的结束：

SELECT 
    DATE, 
    COUNT(CASE 
     WHEN RATING = 1 THEN 1 
     ELSE NULL 
    END) AS STAR1, 
    COUNT(CASE 
     WHEN RATING = 2 THEN 1 
     ELSE NULL 
    END) AS STAR2, 
    COUNT(CASE 
     WHEN RATING = 3 THEN 1 
     ELSE NULL 
    END) AS STAR3, 
    COUNT(CASE 
     WHEN RATING = 4 THEN 1 
     ELSE NULL 
    END) AS STAR4, 
    COUNT(CASE 
     WHEN RATING = 5 THEN 1 
     ELSE NULL 
    END) AS STAR5 
FROM 
    table1 
WHERE 
    DATE = '2012-10-25' 
GROUP BY DATE 
ORDER BY DATE 

请让我知道你是怎么用这种方法做的。

Answer 2

SELECT 
DISTINCT DATE, 
(SELECT COUNT(RATING) FROM Table1 WHERE RATING = '1') AS Star1, 
(SELECT COUNT(RATING) FROM Table1 WHERE RATING = '2') AS Star2, 
(SELECT COUNT(RATING) FROM Table1 WHERE RATING = '3') AS Star3, 
(SELECT COUNT(RATING) FROM Table1 WHERE RATING = '4') AS Star4, 
(SELECT COUNT(RATING) FROM Table1 WHERE RATING = '5') AS Star5 
FROM 
Table1 
WHERE 
DATE = '2012-10-25' 
GROUP BY DATE 
ORDER BY DATE