SQL 获取未参与项目的姓名 [英] SQL get names who didnt work on a project

问题描述

我想检索没有在 PostgreSQL 中参与项目的人员的姓名.

I would like to retrieve the names of people who didn't work on a project in PostgreSQL.

我有一个名为 employees 的表格，其中包含他们的 names 和 ssn.

I got a table named employees with their names and ssn.

我的第二个表名为 works_on，包括 ssn 和 project_number

My second table is called works_on and includes ssn and project_number

ssn 这里是employees

现在我尝试了以下方法:

Now I tried the following:

SELECT fname,lname
FROM werknemer w
JOIN werkt_aan wa 
ON (wa.ssn = w.ssn)
WHERE wa.ssn <> w.ssn

但这不会返回任何内容，但我需要 1 个不在单个项目上工作的名字.有人可以向我解释如何做到这一点吗?提前致谢.

But this returns nothing, but I need 1 name who is not working on a single project. Can someone explain to me how to do this? Thanks in advance.

推荐答案

这通常使用 NOT EXISTS 查询来解决:

This is usually solved using a NOT EXISTS query:

select e.*
from employees e
where not exists (select *
                  from works_on wo
                  where wo.ssn = e.ssn)

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