Question

我有student_uid，grade，test_name作为列的表我想计算每个等级有多少...这个

SELECT a.grade,COUNT(a.grade) AS count1 
FROM 2015_2016_x_english_grades AS a 
where test_name='ut1_marks' 
GROUP BY grade

对于单个表，如何为多个表执行此操作

我的疑问：

SELECT a.grade, COUNT(a.grade),b.grade,COUNT(b.grade) 
FROM 2015_2016_x_english_grades a 
INNER JOIN 2015_2016_x_hindi_grades b ON a.grade=b.grade 
WHERE a.test_name = b.ut1_marks='ut1_marks' 
GROUP BY a.grade,b.grade

这有什么问题？我也尝试过这个 SELECT a.grade，COUNT（a.grade），（SELECT COUNT（b.grade）FROM 2015_2016_x_biology_grades b其中b.test_name ='ut1_marks'GROUP BY b.grade）as count1 FROM 2015_2016_x_biology_grades a where test_name ='ut1_marks'GROUP BY一个品级它说[Err] 1242 - 子查询返回超过1行

Answer 1

在子查询中进行计数，并加入子查询。

SELECT e.grade, english_count, hindi_count
FROM (SELECT grade, COUNT(*) AS english_count
      FROM 2015_2016_x_english_grades
      WHERE test_name = 'ut1_marks'
      GROUP BY grade) AS e
JOIN (SELECT grade, COUNT(*) as hindi_count
      FROM 2015_2016_x_hindi_grades
      WHERE test_name = 'ut1_marks'
      GROUP BY grade) AS h
ON e.grade = h.grade

或者，如果每个表中都有唯一的密钥，您可以执行以下操作：

SELECT e.grade, COUNT(DISTINCT e.id) AS english_count, COUNT(DISTINCT h.id) AS hindi_count
FROM 2015_2016_x_english_grades AS e
JOIN 2015_2016_x_hindi_grades AS h ON e.grade = h.grade AND e.test_name = h.test_name
WHERE e.test_name = 'ut1_marks'
GROUP BY e.grade

请注意，如果两个表中都存在等级，则这两个查询都只显示等级。要获得仅存在于一个表中的成绩，您需要FULL OUTER JOIN，但MySQL没有此操作。参见

Full Outer Join in MySQL

如何模仿它们。

多个表中的多个计数，也按年级分组

1 个答案: