Question

[1]：https://i.stack.imgur.com/R5Pzz.png我目前正在尝试列出每个assignmentID的最小值，最大值，平均值和计数。我用来创建图片中显示的代码如下：

SELECT assignment.assignmentID, assignment.assignmentName, grade.grade

FROM grade 

JOIN assignment ON grade.assignmentID = assignment.assignmentID

当我将最小值，最大值，平均值，计数添加到SELECT函数时，它只返回赋值ID的结果1.如何让它为所有赋值执行此操作

Answer 1

那是因为你没有按照作业记录进行分组。使用group by在每个分配的基础上执行聚合：

select assignment.assignmentID, assignment.assignmentName,
    count(*) as gradeCount, avg(grade.grade) as avgGrade,
    min(grade.grade) as minGrade, max(grade.grade) as maxGrade
from assignment, grade
where assignment.assignmentID = grade.assignmentID
group by assignment.assignmentID, assignment.assignmentName

Answer 2

我相信你想要：

SELECT a.assignmentID, a.assignmentName,
       COUNT(*) as numgrades, MIN(g.grade), MAX(g.grade), AVG(g.grade)
FROM assignment a JOIN
     grade g
     ON g.assignmentID = a.assignmentID
GROUP BY a.assignmentID, a.assignmentName;

您的查询缺少GROUP BY和聚合函数。这假定grade是数字，因此AVG()有意义。

请注意，我还引入了表别名，因此查询更容易编写和阅读。

SQL：列出min，max，average，count

2 个答案: