Question

对在所有部门工作的员工的查询是什么？这里的部门和员工有很多基数。

这些表是：

CREATE TABLE employees
(
    employee_id int NOT NULL CONSTRAINT pk_employees PRIMARY KEY,
    employee_name nvarchar(128) NOT NULL CONSTRAINT uk_employees_employee_name UNIQUE
);

CREATE TABLE departments
(
    department_id int NOT NULL PRIMARY KEY,
    department_name nvarchar(128) NOT NULL CONSTRAINT uk_departments_department_name UNIQUE
);

CREATE TABLE department_employees
(
    department_id int NOT NULL CONSTRAINT fk_department_employees_departments REFERENCES departments(department_id),
    employee_id int NOT NULL CONSTRAINT fk_departement_employees_employees REFERENCES employees(employee_id),
    CONSTRAINT pk_deparment_employees PRIMARY KEY (department_id, employee_id)
)

样本数据：

INSERT INTO employees
VALUES (1, 'John Doe'), (2, 'Jane Doe'), (3, 'William Doe'), (4, 'Margaret Doe')

INSERT INTO departments
VALUES (1, 'Accounting'), (2, 'Humman Resources'), (3, 'Marketing')

INSERT INTO department_employees
VALUES 
    (1, 1), (2, 1), (3, 1),
    (2, 2), (2, 3),
    (3, 3), (3, 4)

预期结果：

+-------------+---------------+
| employee_id | employee_name |
+-------------+---------------+
|           1 | John Doe      |
+-------------+---------------+

Answer 1

在关系代数上此操作称为Relation Division:。

可以使用以下查询在sql中实现

SELECT *
FROM dbo.employees e
WHERE 
    NOT EXISTS (
        SELECT *
        FROM departments d
        WHERE d.department_id NOT IN (
            SELECT dp.department_id
            FROM department_employees dp
            WHERE dp.department_id = d.department_id AND dp.employee_id = e.employee_id
        )
    )

请注意，查询“给我所有部门的员工”等同于“给所有雇员没有该员工不在的部门”

Answer 2

您可以在下面的查询中尝试使用此方法。

在一个变量中，所有不同的计数部门均已从部门主表中获取。之后，仅选择那些计数与关联表中不同的链接部门计数匹配的员工。

declare @distinctDeptCount int 
SET @DistinctDeptCount = (SELECT Count(Distinct department_id) FROM departments)
--SELECT @DistinctDeptCount

SELECT Distinct employees.employee_id, employee_name
from employees     
where employees.employee_id in (
select employee_id from department_employees GROUP BY employee_id HAVING COUNT(department_id) = @distinctDeptCount
)

OR

SELECT Distinct employees.employee_id, employee_name
from employees     
where employees.employee_id in (
select employee_id from department_employees GROUP BY employee_id HAVING COUNT(department_id) = 
(SELECT Count(Distinct department_id) FROM departments)
)

输出如下所示

employee_id employee_name
1           John Doe

这是现场演示Emp. with all departments

Answer 3

您可以尝试

SELECT
    emp.name
FROM 
    Employee emp 
JOIN 
    EmpDept empd 
ON 
    emp.EmployeeID = empd.EmpId 
GROUP BY
    emp.EmployeeID
HAVING 
    count(emp.EmployeeID) = (select count(1) from Department)

对于在所有部门工作的员工，该查询是什么？

3 个答案: