Question

我需要列出客户名称及其项目名称，其中有员工GLC处理任务而不是员工CAC处理任务。**

我尝试了以下内容，但得到了一个空集：

select 
   c.custname, p.pjtitle
from 
   (select taskno pjno
    from task 
    where taskno not in
                 (select taskno
                  from task
                  where empid = 'CAC')
    and empid = 'GLC') t
join project p on p.pjno = t.pjno
join customer c on p.custno = c.custno;

我应该如何正确使用join和subquery？

Answer 1

SELECT c.custname, p.pjtitle
  FROM customer c
  JOIN project p ON p.custno = c.custno
  JOIN task t ON p.pjno=t.pjno
 WHERE t.empid='GLC'
   AND NOT EXISTS (SELECT NULL FROM task t1 WHERE t.pjno=t1.pjno AND t1.empid='CAC')
;

Answer 2

如果您需要客户名称和项目名称，那么您可能不希望重复。这是set-within-sets子查询的示例。我认为最好的解决方案是使用聚合，因为这为描述条件提供了最大的灵活性。

SELECT c.custname, p.pjtitle 
FROM customer c join
     project P
     ON P.CUSTNO = P.CUSTNO join
     task t ON t.pjno = p.pjno
group by c.custname, p.pjtitle 
having sum(case when empid = 'GLC' > 0 then 1 else 0 end) > 0 and
       sum(case when empid = 'CAC' > 0 then 1 else 0 end) = 0

having子句的两个部分是计算每个员工在特定项目上工作的次数。第一个说“确保GLC正在进行至少一次任务”，第二个说“确保CAC正在进行任务”。

Answer 3

如果您只需要GLC员工的客户名称和项目，那么请使用以下查询

SELECT  c.custname ,
        p.pjtitle
FROM    Customer AS c
        INNER JOIN project AS p ON c.CUSTNO = p.CUSTNO
        INNER JOIN task AS t ON p.PJNO = t.PJNO
WHERE   t.EMPID = 'GLC'

如果您想要员工GLC和CAC，那么

SELECT  c.custname ,
        p.pjtitle
FROM    Customer AS c
        INNER JOIN project AS p ON c.CUSTNO = p.CUSTNO
        INNER JOIN task AS t ON p.PJNO = t.PJNO
WHERE   t.EMPID = 'GLC'
        AND t.EMPID = 'CAC'

Answer 4

试试这个：

SELECT c.custname, p.pjtitle 
FROM customer AS c 
JOIN project AS P ON P.CUSTNO = P.CUSTNO
JOIN tack AS t ON t.pjno = p.pjno
WHERE t.empid = 'GLC'
AND NOT EXISTS (SELECT NULL FROM TASK WHERE empid = 'GLC' AND empid = 'CAC');

Answer 5

检查此答案是否有效

select c.custname, p.pjtitle 
  from project p, customer c where c.custno = p.custno 
and exists
(select t.pjno from task t where t.empid = 'GLC' and p.pjno = t.pjno 
and   t.pjno not IN (select pjno from task where empid = 'CAC'))

SQL。如何组合子查询并正确连接？

5 个答案: