来自多个表的多个计数，也按年级分组

Question

我有 table 和 student_uid,grade,test_name 作为列我想计算每个等级有多少..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 table 工作如何为多个 table

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？我也试过这个 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 a.grade 它说 [Err] 1242 - 子查询 returns 超过 1 行

Answer 1

在子查询中做计数，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 table 中都有一个唯一的键，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，如果成绩同时存在于 table 中，这两个查询只会显示成绩。要获得只存在于一个 table 中的成绩，您需要一个 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。

来自多个表的多个计数，也按年级分组

multiple count from multiple tables and also group by grade

mysql

count