Question

我正在尝试查询列出顾问并提供每个活跃学生的数量。我可以让它列出有1名学生的顾问，排除那些超过1的学生，但不能让它以0或NULL计数返回顾问。

Select Advisors.AdvisorID, Advisors.FirstName, Advisors.LastName, COUNT(case Students.IsActive WHEN '1' then 1 else NULL end) AS "Number of Students"
FROM Advisors, Students
WHERE Advisors.AdvisorID=Students.AdvisorID
GROUP BY Advisors.AdvisorID, Advisors.FirstName, Advisors.LastName
HAVING COUNT(case Students.IsActive WHEN '1' then 1 else NULL end)='1'

计算活跃的学生，并将顾问列表与顾问一起返回给一名学生，0名学生的顾问将空白。我错过了什么？

Select Advisors.AdvisorID, Advisors.FirstName, Advisors.LastName, COUNT(case Students.IsActive WHEN '1' then 1 else NULL end) AS "Number of Students"
FROM Advisors, Students
WHERE Advisors.AdvisorID=Students.AdvisorID
GROUP BY Advisors.AdvisorID, Advisors.FirstName, Advisors.LastName
HAVING COUNT(case Students.IsActive WHEN '1' then 1 else NULL end) IS NULL

返回列名，没有数据。我已经仔细检查了表顾问表有3个条目，一个有2个活跃学生，一个非活动0或1使用位，一个没有学生，一个有一个。

使用＆lt; = 1或＆lt; 1类似地导致空白数据。

Answer 1

请使用ANSI JOIN语法

Select    Advisors.AdvisorID, 
          Advisors.FirstName, 
          Advisors.LastName, 
          COUNT(case Students.IsActive WHEN '1' then 1 else NULL end) AS "Number of Active Students"
FROM      Advisors
LEFT JOIN Students
ON        Advisors.AdvisorID=Students.AdvisorID
GROUP BY  Advisors.AdvisorID, 
          Advisors.FirstName, 
          Advisors.LastName
HAVING COUNT (Students.AdvisorID) = 1

SQL Query返回一个结果，不返回0或NULL结果

1 个答案: