Question

我有两个桌子

表1：违规列：日期，时间，pdid，pname，v1，v2，v3，v4

每个v1到v4具有一个整数值，该整数值对应于表2中的单个条目（ID）。

表2：parking_violations 列：代码，部分，说明，ID

我需要基于pdid查询每个违规记录，并将每个“ v1-v4”与p_violations表中的“ ID”列进行匹配。

SELECT 
    parking.date,
    parking.time,
    parking.pname,
    parking_violations.code,
    parking_violations.section,
    parking_violations.description
FROM 
    parking
INNER JOIN parking_violations ON parking.v1=parking_violations.ID
WHERE 
    pdid=5

这将返回V1的正确记录，但是我无法弄清楚如何还通过将值与ID匹配来返回所有填充的V2-V4。

Answer 1

使用别名多次连接，如下所示：

SELECT 
    parking.date,
    parking.time,
    parking.pname,
    parking_violations.code,
    parking_violations.section,
    parking_violations.description
FROM 
    parking
INNER JOIN parking_violations ON parking.v1=parking_violations.ID
inner join parking_violations a ON parking.v2=a.ID
inner join parking_violations b ON parking.v3=b.ID
inner join parking_violations c ON parking.v4=d.ID
WHERE 
    pdid=5

Answer 2

就像@ fa06解释的那样，您可以对同一张表使用多个联接，但是我将使用左联接，而不是内部联接，这样我就可以灵活地获取并非所有 vN 都在表parking_violations上具有匹配ID的行。

SELECT 
    parking.date,
    parking.time,
    parking.pname,
    pv1.code,
    pv1.section,
    pv1.description,
    pv2.code,
    pv2.section,
    pv2.description,
    pv3.code,
    pv3.section,
    pv3.description,
    pv4.code,
    pv4.section,
    pv4.description
FROM
    parking
LEFT JOIN
    parking_violations AS pv1 ON pv1.ID = parking.v1
LEFT JOIN
    parking_violations AS pv2 ON pv2.ID = parking.v2
LEFT JOIN
    parking_violations AS pv3 ON pv3.ID = parking.v3
LEFT JOIN
    parking_violations AS pv4 ON pv4.ID = parking.v4
WHERE 
    parking.pdid = 5;

MySQL INNER JOIN多列

2 个答案: