Question

我已经在Stack Overflow上看到了几篇关于此的文章，但是似乎都没有一个我能理解的答案。

我正在尝试将几个关系合并在一起，以获取所有相关信息，以输出从中国开始到美国结束的所有路线。

在SeaRoute关系中，start_port和end_port被存储为INT，在Port关系中，pid对应于start_port和end_port，并包含一个pcountry列。

我首先尝试输出在中国具有start_port的所有内容。我期望Record关系带来3个结果，因为这些是表中唯一以中国开头的结果；但是，我在输出处收到6条记录（如果我返回并审核表中的内容，所有结果似乎都翻了一番。）

尽管我想要正确的答案，但我更担心我对Inner Join和其他Join方法有基本的误解。我在做什么错了？

SELECT *
FROM Record
INNER JOIN Goods AS Go_data 
    ON Record.gid = Go_data.gid
LEFT JOIN SeaRoute AS SR 
    ON Record.rid = SR.rid
RIGHT JOIN (SELECT pid, pcountry AS starting_port_country
            FROM Port
            INNER JOIN SeaRoute AS SR ON Port.pid = SR.start_port
            WHERE Port.pcountry = 'China') 
            AS start_port_table ON SR.start_port = start_port_table.pid

Answer 1

从查询的外观来看，您希望在仅拥有所需路线的记录之间进行内部联接。

您知道所有始于中国并已结束于美国的SeaRoute，但是您确实需要像这样两次连接到Ports表：

SELECT  sr.rid,
        sp.pcountry AS starting_port_country,
        ep.pcountry AS end_port_country
FROM dbo.SeaRoute sr
    INNER JOIN dbo.Port sp ON sp.pid = sr.start_port
    INNER JOIN dbo.Port ep ON ep.pid = sr.end_port
WHERE   sp.pcountry = 'China'
    AND ep.pcountry = 'United States'

然后，您只需要将其加入到主查询中即可：

SELECT *
FROM Record
    INNER JOIN dbo.Goods AS Go_data     ON Record.gid = Go_data.gid
    INNER JOIN  
        (
            SELECT  sr.rid,
                    sp.pcountry AS starting_port_country,
                    ep.pcountry AS end_port_country
            FROM dbo.SeaRoute sr
                INNER JOIN dbo.Port sp ON sp.pid = sr.start_port
                INNER JOIN dbo.Port ep ON ep.pid = sr.end_port
            WHERE   sp.pcountry = 'China'
                AND ep.pcountry = 'United States'
        ) ports ON ports.rid = Record.rid

没有任何方法可以比此页面更清楚地向您解释联接：

https://www.codeproject.com/Articles/33052/Visual-Representation-of-SQL-Joins

连接加倍结果的问题

1 个答案: