Question

我有一张关于员工及其经理的简单表格。我想显示两列，一列具有雇员姓名，另一列具有雇员的经理姓名，如果雇员没有经理，则表应返回null。

我可以使用SELF JOIN来提取经理姓名，但是很难将其用作列。

这是表结构：

CREATE TABLE employees (
id INTEGER NOT NULL PRIMARY KEY,
mgrId INTEGER REFERENCES employees(id),
name VARCHAR(30) NOT NULL
);

INSERT INTO employees(id, mgrId, name) VALUES(1, NULL, 'Joey');
INSERT INTO employees(id, mgrId, name) VALUES(2, 1, 'Ross');
INSERT INTO employees(id, mgrId, name) VALUES(3, 1, 'Chandler');
INSERT INTO employees(id, mgrId, name) VALUES(4, 2, 'Monica');

这是我的解决方案，但输出显示JoeY为所有人的经理：

SELECT name, (SELECT e1.name
          FROM employees as e1 JOIN employees as e2
          ON e1.id = e2.mgrId) AS manager
FROM employees

这是我要打印的内容（Joey没有管理员，因此为null）：

NAME        Manager
----------  -------
Joey
Ross        Joey
Chandler    Joey
Monica      Ross

Answer 1

您需要外部联接才能产生所需的结果：

select
  e.name,
  m.name as manager
from employees e
left join employees m on m.id = e.mgrid
order by e.name

请参见DB Fiddle上的运行示例。

Answer 2

您的子查询与您的主查询没有关联，这就是为什么它为每个人返回相同结果的原因。将其更改为关联的子查询将解决您的问题：

SELECT e2.name, (SELECT name FROM employees as e1 WHERE e1.id = e2.mgrId) AS manager
FROM employees e2

输出：

name        manager
Joey        null
Ross        Joey
Chandler    Joey
Monica      Ross

Demo on dbfiddle

Answer 3

您刚刚在内部查询中做了一个额外的联接，该联接应该是：

SELECT name, (SELECT e1.name
          FROM employees as e1
          WHERE e1.id = e2.mgrId) AS manager 
FROM employees e2

Answer 4

我是否可以建议对数据进行不同的建模（请查看Vanilla Peoplesoft HR数据模型以获取灵感），但为简单起见，我仅添加了一个位置表，以下查询提供了具有更大灵活性的预期结果：< / p>

Select e.id, e.positionid, e.name, p.positionid_reports_to, m.name as Manager
FROM employee e
Left Join position p on e.positionid = p.positionid
Left Join employee m on p.positionid_reports_to = m.positionid;

基于以下数据：

INSERT INTO employee(id, positionId, name) VALUES(1, 101, 'Boss');
INSERT INTO employee(id, positionId, name) VALUES(2, 102, 'Joey');
INSERT INTO employee(id, positionId, name) VALUES(3, 103, 'Ross');
INSERT INTO employee(id, positionId, name) VALUES(4, 104, 'Chandler');
INSERT INTO employee(id, positionId, name) VALUES(5, 105, 'Monica');


INSERT INTO position(positionid, positionid_reports_to) VALUES(101, null);
INSERT INTO position(positionid, positionid_reports_to) VALUES(102, 101);
INSERT INTO position(positionid, positionid_reports_to) VALUES(103, 101);
INSERT INTO position(positionid, positionid_reports_to) VALUES(104, 101);
INSERT INTO position(positionid, positionid_reports_to) VALUES(105, 101);

有没有一种方法可以将SELECT语句用作列？

4 个答案: