sql - 嵌套查询中的 SQL Server 查询更正

Question

SELECT campus,semester, AVG(CountOfStudents)
FROM
(
  SELECT semester,year,campus, count(*) as CountOfStudents
  FROM regestration
  GROUP BY  semester, year, campus,student_id
) t
GROUP BY   campus,semester

我需要做的是找到每个校区每学期的平均人数

我的表结构是：

表名 -注册

student_id
campus
year
batch
semester

campus, year,semester和batch这些可以帮助识别唯一记录，其中student_id可能会重复上面的查询给出错误的答案。

Answer 1

跟着这些步骤：

• student_ID从GROUP BY子句中删除和
• 添加DISTINCT里面COUNT()

询问，

SELECT campus, semester, AVG(CountOfStudents)
FROM
(
  SELECT semester, year, campus, count(DISTINCT student_id) as CountOfStudents
  FROM   registration
  GROUP BY  semester, year, campus
) t
GROUP BY   campus, semester