MYSQL使用JOIN从一个表中拉出两列

Question

我正在尝试使用JOIN，但似乎我只获得了一半的结果。以下是表格和所需输出。

我没有收到任何错误 当我使用JOIN时，它只显示approver1。所以我做的是：

SELECT tb1.*, tb2.name AS a1, tb3.name AS a2 
FROM   table1 tb1 
       LEFT JOIN approver tb2 
       ON tb1.approve1 = tb2.id 
       LEFT JOIN approver tb3 
       ON tb1.approver2 = tb2.id

非常感谢任何帮助或线索。

Answer 1

您必须使用LEFT JOIN来获取没有关联审批者的行。例如。 Dwade Curtis没有关联的approver2

SELECT e.emp_name, a1.name as ap1, a2.name as ap2
FROM employee e 
LEFT JOIN approver a1 ON (a1.id=e.approver1)
LEFT JOIN approver a2 ON (a2.id=e.approver2)

This是对SQL JOIN的良好直观解释

Answer 2

Select empname, ap_1.name, ap_2.name from Employee 
left join Approver as ap_a on (Employee.approver1 = ap_1.id)
left join Approver as ap_b on (Employee.approver2 = ap_2.id)