mysql – SQL:在SUM中放置一个Distinct

我有一张桌子可用:

name |test_date | score   
-----------------------
jon  |2012:07:01|   95   
jon  |2012:07:01|   60   
jon  |2012:07:01|   30   
alex |2012:07:01|   80   
alex |2012:07:01|   85   
alex |2011:05:01|   40  
emile|2011:01:01|   89    

我想为每个名称获取一些行,并通过规则提供有关其score_grading的信息:
得分> 79 = A,
80 GT;得分> 49 = B
否则C.

问题是,我想:如果在同一天,学生有多个相同的score_grade,那么它将被计为一个score_grade.
例如,在表格中我们可以看到alex在一天内获得A两次,我希望它只被计为1 A.

结果将是

name |    A   |    B     |    C
jon  |    1   |    1     |    1
alex |    1   |    0     |    1  
emily|    1   |    0     |    0

我只知道代码:

SELECT name,
SELECT SUM(IF(score)>79),1,0)) as A,
SELECT SUM(IF(80>score>49),1,0)) as B,
SELECT SUM(IF(score)<50),1,0)) as C from sometable group by name

不,我怎么把“DISTINCT”放在上面?
任何人都可以提供解决方案吗?也许它不需要DISTINCT?
谢谢. ^^

最佳答案 你的意思是如下(
sqlfiddle)?

SELECT name, SUM(IF(A>=1,1,0)) as A, SUM(IF(B>=1,1,0)) as B, 
    SUM(IF(C>=1,1,0)) as C
FROM
(
    SELECT name, test_date, SUM(IF(score>79,1,0)) as A, 
        SUM(IF(score BETWEEN 49 AND 79,1,0)) as B, SUM(IF(score<50,1,0)) as C 
    FROM sometable 
    GROUP BY name, test_date
) daygroups
GROUP BY name

这首先将您的数据混合到子查询中的名称,test_date,A,B,C行.然后外部查询将聚合这些行,如果当天至少有一个该字母等级的分数,则取1,否则取0.

这应该也有效(sqlfiddle):

SELECT name, SUM(IF(lettergrade = 'A',1,0)) AS A, 
    SUM(IF(lettergrade = 'B',1,0)) AS B, SUM(IF(lettergrade = 'C',1,0)) AS C
FROM
(
    SELECT DISTINCT name, test_date, 
        CASE WHEN score>79 THEN 'A' 
             WHEN score BETWEEN 49 AND 79 THEN 'B' 
             ELSE 'C' 
        END AS lettergrade
    FROM sometable
) lettergrades
GROUP BY name

我不确定哪一个更好.此查询使用您在问题中建议的DISTINCT.首先,它将每个数字等级解析为相应的字母等级,然后DISTINCTs重复.最后,它将数据混合到列中.

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