SQL分组依据并计入各科目的成绩总计

Question

我还没有找到一个完全符合我想要的答案的答案。

老实说，我不太确定该怎么做！

我有一张表格，列出各学科的学生取得的成绩：

我想要一张表格，该表格显示每个科目的成绩分布情况，以及每个授予的成绩总数：

到目前为止，我的查询是：

SELECT subjectcode,
(SELECT COUNT(grade) FROM results WHERE grade = "A") as A,
(SELECT COUNT(grade) FROM results WHERE grade = "B") as B,
(SELECT COUNT(grade) FROM results WHERE grade = "C") as C,
(SELECT COUNT(grade) FROM results WHERE grade = "D") as D,
(SELECT COUNT(grade) FROM results WHERE grade = "E") as E
FROM results
GROUP BY grade
ORDER BY subjectcode ASC;

任何意见或建议，将不胜感激。

谢谢。

Answer 1

使用条件聚合，而不是多个子查询：

SELECT subjectcode,
       SUM(CASE WHEN grade = 'A' THEN 1 END) as A,
       SUM(CASE WHEN grade = 'B' THEN 1 END) as B,
       SUM(CASE WHEN grade = 'C' THEN 1 END) as C,
       SUM(CASE WHEN grade = 'D' THEN 1 END) as D,
       SUM(CASE WHEN grade = 'E' THEN 1 END) as E
FROM results
GROUP BY subjectcode
ORDER BY subjectcode ASC;

笔记：

• group by子句是固定的。 您在group by grade依据group by ，但subjectcode进行排序，这没有意义。 大多数数据库甚至都不允许这样做。
• 在这种case没有else子句，因此不匹配将产生NULL 。
• 字符串常量由单引号而不是双引号分隔。 单引号是ANSI标准，应用于字符串和日期常量。
• 这称为数据透视查询，某些数据库具有直接支持它们的语法。

Answer 2

declare @grades table(result_id int identity, upn int, grade varchar(2), subjectcode varchar(2))

insert into @grades (upn,grade,subjectcode) values
(1,'D','Ar'),
(1,'A','Bi'),
(1,'C','Ch'),
(2,'A*','Ar'),
(2,'B','Ch'),
(2,'A*','PE'),
(3,'C','Ar'),
(3,'C','Ph'),
(3,'A','Bi')


SELECT subjectcode,
    ISNULL(SUM(CASE WHEN grade = 'A*' THEN 1 END),'') 'A*',
    ISNULL(SUM(CASE WHEN grade = 'A' THEN 1 END),'') A,
    ISNULL(SUM(CASE WHEN grade = 'B' THEN 1 END),'') B,
    ISNULL(SUM(CASE WHEN grade = 'C' THEN 1 END),'') C,
    ISNULL(SUM(CASE WHEN grade = 'D' THEN 1 END),'') D,
    ISNULL(SUM(CASE WHEN grade = 'E' THEN 1 END),'') E,
    ISNULL(SUM(CASE WHEN grade = 'F' THEN 1 END),'') F,
    ISNULL(SUM(CASE WHEN grade = 'G' THEN 1 END),'') G,
    ISNULL(SUM(CASE WHEN grade = 'U' THEN 1 END),'') U,
    ISNULL(SUM(CASE WHEN grade = 'X' THEN 1 END),'') X

FROM @grades
GROUP BY subjectcode
ORDER BY subjectcode ASC;