SQL 分组依据和计数科目的成绩总计

Question

我还没有找到与我想做的事情完全匹配的答案。

说实话，我并不完全确定如何去做!

我有一个表格，其中包含学生在每个科目中取得的成绩:

我想要一个表格，显示每个科目的成绩分布，每个成绩的总和:

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;

如有任何意见或建议，我们将不胜感激。

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;

注释:

• group by 子句已修复。您在 group by 中有 grade，但却按 subjectcode 排序，这是没有意义的。这在大多数数据库中甚至是不允许的。
• case 上没有 else 子句，因此不匹配会产生 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。