同桌加入

Question

表员工有ssn和superssn coloumns。我想显示每个员工的主管的主管（如果不存在，则为null）。到目前为止，我只有每位员工的主管：

SELECT p.fname AS sup,
group_concat(c.fname) AS emp 
FROM employee AS p
LEFT JOIN employee AS c 
ON (p.ssn = c.superssn) 
GROUP BY p.ssn;

我应该再做一次JOIN吗？或GROUP BY中的某些内容？

Answer 1

如果您想要经理，员工和经理的主管，那么您可以多次加入表employee：

SELECT p.fname AS sup,   -- Manager
    group_concat(c.fname) AS emp,  -- Employees
    sp.fname as SuperSuper   -- Manager Supervisor
FROM employee AS p
LEFT JOIN employee AS c 
    ON p.ssn = c.superssn
LEFT JOIN employee sp
    on p.superssn = sp.ssn
GROUP BY p.ssn;