SQL数据库查询：计数（不同）

Question

我正在进行SQL查询以执行以下操作：

  

对于两个以上员工工作的每个项目，检索项目编号，项目名称和从事该项目的员工人数。

这就是我目前对项目1的目的：

SELECT project.PNO, project.PNAME, COUNT(DISTINCT works_on.ESSN)
AS '# of employess worked on'

FROM project INNER JOIN works_on
ON project.PNO = works_on.PNO

INNER JOIN employee
ON works_on.ESSN = employee.SSN

HAVING COUNT(DISTINCT works_on.ESSN) > 2
AND project.PNO LIKE 1

然后它返回：

PNO：1

PNAME：ProductX

就业人数：7

当它应该返回NULL时，因为项目编号1在works_on表中只有两个不同的ESSN，而是返回works_on表中不同ESSN的总数

附件是我的架构：

Answer 1

您需要按项目获得结果，这意味着您错过了group by条款：

SELECT     project.PNO,
           project.PNAME, 
           COUNT(DISTINCT works_on.ESSN) AS '# of employess worked on'
FROM       project 
INNER JOIN works_on ON project.PNO = works_on.PNO
INNER JOIN employee ON works_on.ESSN = employee.SSN
GROUP BY   project.PNO, project.PNAME -- Here!
HAVING     COUNT(DISTINCT works_on.ESSN) > 2 AND project.PNO LIKE 1

Answer 2

使用Group by和where子句，如下所示：

SELECT project.PNO, project.PNAME, COUNT(DISTINCT works_on.ESSN)
AS '# of employess worked on'

FROM project INNER JOIN works_on
ON project.PNO = works_on.PNO

INNER JOIN employee
ON works_on.ESSN = employee.SSN
where project.PNO LIKE 1
group by project.PNO, project.PNAME 
HAVING COUNT(DISTINCT works_on.ESSN) > 2