需要三张表的组合查询 SQL Query

Question

我发现解决以下问题有点令人困惑......

" 一名员工可能被分配到多个项目，而一个项目可能有许多员工。考虑以下关系模式并为以下查询编写 SQL 语句。

Employees (empID, empName, empDOB, empAddress, salary, deptID, jobID)

Assignments (empID, projID, assignedDate, completionDate, status)`

Projects (projID, projDescription, startDate, endDate, projType)

(a) 显示 1980 年 1 月 31 日之前出生并分配了“办公综合体”类型项目的员工姓名，按姓名升序排列结果。

(b) 检索分配了至少两 (2) 个项目的 empID。 "

到目前为止我的回答。

(a)SELECT empName FROM EMPLOYEES WHERE empDOB < '31-january-1980' AND ....

"

请帮帮我

Answer 1

试试这个： 一）

SELECT employee.empName
FROM Assignments ass 
   JOIN Employees employee
      ON employee .empID= ass.empID
   JOIN Projects  project
      ON project.projID= ass .projID
WHERE employee.empDOB < '31-january-1980' 
      AND project.projType = ‘Office Complex’
ORDER BY  employee.empName;

不使用JOIN CLAUSE

SELECT DISTINCT employee.empName
FROM Employees employee
WHERE employee.empDOB < '31-january-1980' 
      AND employee.empID  IN (
           SELECT ass.empID
           FROM Assignments ass
           WHERE ass.projID  IN (
                SELECT project.projID
                FROM  Projects project
                WHERE project.projType = ‘Office Complex’
           )
      )
ORDER BY  employee.empName ASC;

b)

SELECT ass.empID, COUNT(*) as counting 
FROM Assignments ass
GROUP BY ass.empID
HAVING COUNT(*) >= 2;