【MySQL】外连接的曲线救国

因为mysql 不支持外连接，所以当需要用到外连接的时候，这题的解法就相当有趣，记录一下。

leetcode 1965：写出一个查询语句，找到所有丢失信息的雇员id。

力扣 1965

题解

（1）使用union和left join曲线解决

分别使用两张表进行左连接，将左连接的结果合并，筛选出空值

SELECT employee_id from(
    SELECT t1.employee_id, t1.name, t2.salary FROM Employees t1 LEFT JOIN Salaries t2 ON t1.employee_id = t2.employee_id 
    UNION 
    SELECT t2.employee_id, t1.name, t2.salary FROM Employees t1 RIGHT JOIN Salaries t2 ON t1.employee_id = t2.employee_id)t 
    where name is null or salary is null order by employee_id asc

（2）使用union和not in/exists曲线解决

分别找出两张表的非交集部分，然后结果合并

SELECT  employee_id from(
    SELECT employee_id from Employees where employee_id not in(SELECT employee_id from Salaries 
    UNION 
    SELECT employee_id from Salaries where employee_id not in(SELECT employee_id from Employees))t 
    order by employee_id asc

（3）使用union和having曲线解决

合并两张表，缺少数据的id应该只出现一次

select employee_id from (
    select employee_id from employees 
    union all 
    select employee_id from salaries)t 
    group by employee_id having count(employee_id) = 1 
    order by employee_id asc