mysql - 来自多个表的多次计数并按年级分组

Question

我有一个以student_uid、grade、test_name为列的表，我想计算每个等级有多少人......为此

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

我的查询:

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 

这有什么问题吗？我也试过这个SELECT a.grade,COUNT(a.grade),(SELECT COUNT(b.grade)FROM 2015_2016_x_biology_grades b where 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 它显示 [Err] 1242 - 子查询返回超过 1 行

Answer 1

在子查询中进行计数，并连接子查询。

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，如果每个表中有一个唯一的键，您可以这样做:

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，这两个查询只会显示两个表中都存在的成绩。要获得只存在于一张表中的成绩，需要使用FULL OUTER JOIN，但MySQL没有此操作。参见

Full Outer Join in MySQL

了解如何模拟它们。