SQL查询提取具有不同值的相同字段答案

【问题标题】：SQL Query pulling same field with different valuesSQL查询提取具有不同值的相同字段
【发布时间】：2015-04-18 00:12:40
【问题描述】：

我有两个表，员工记录（id、firstName、lastName）和违规记录（violatorId、supervisorId）。

我的查询提取了违规者和主管的列表。我想显示他们的名字和姓氏，而不是显示此人的 ID。所以这是我目前的查询......

SELECT 
emp.firstName as vFirstName,
emp.lastName as vLastname,
v.violatorId, 
v.supervisorId,
emp.firstName as sFirstName,
emp.lastName as sLastName
FROM employee emp
LEFT JOIN violator v
ON violatorId = emp.id

因此，这将返回违规者的名字和姓氏信息，但不会返回主管。样本输出：

Violator vFirstName  vLastName   Supervisor sFirstName sLastName
1        Jon         Smith       20         Jon        Smith

任何帮助都会很棒

【问题讨论】：

您的 supervisorId 是否引用了 Employee 表？

标签： mysql sql

【解决方案1】：

SELECT 
  e1.firstName AS violatorFirstName,
  e1.lastName AS violatorLastName,
  e2.firstName AS supervisorFirstName,
  e2.lastName AS supervisorLastName
FROM violator v
LEFT JOIN employee e1 ON v.violatorId = e1.id
LEFT JOIN employee e2 ON v. supervisorId = e2.id

【讨论】：

【解决方案2】：

您需要再次加入employee:

SELECT emp.firstName AS vFirstName, emp.lastName AS vLastName,
       v.violatorId AS vId, v.supervisorId AS sId,
       s.firstName AS sFirstName, s.lastName AS sLastName
FROM employee emp
LEFT JOIN violator v ON v.violatorId = emp.id
LEFT JOIN employee s ON v.supervisorId = s.id

【讨论】：

【解决方案3】：

方法一

SELECT 
  e1.violatorId AS violatorId ,
  e1.firstName AS violatorName,
  e2.supervisorId AS supervisorId ,
  e2.firstName AS supervisorName
FROM violator v
LEFT JOIN employee e1 ON v.violatorId = e1.id
LEFT JOIN employee e2 ON v. supervisorId = e2.id

方法 2

SELECT 
    v.violatorId,
    emp.firstName AS violatorName, 
    v.supervisorId,
    (SELECT firstName FROM employee WHERE employee.id = v.supervisorId) AS supervisorName
FROM 
    employee emp
LEFT JOIN 
    violator v
ON 
    violatorId = emp.id

【讨论】：