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 以及聚合函数。这是假设 grade 是数字，所以 AVG() 有道理。
注意，我还引入了表别名，因此查询更易于编写和读取。

那是因为你没有按分配记录分组。使用 group by 要在每个分配的基础上执行聚合，请执行以下操作：

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