SQL INNER JOIN多个表无法按预期工作 [英] SQL INNER JOIN multiple tables not working as expected

问题描述

我试图更好地理解SQL数据库中的JOIN或INNER JOIN多个表。

I'm trying to have a better understanding of JOIN or INNER JOIN multiple tables in a SQL database.

这就是我所拥有的：

SQL查询：

SELECT *
FROM csCIDPull
INNER JOIN CustomerData ON CustomerData.CustomerID = csCIDPull.CustomerID
    INNER JOIN EMSData ON EMSData.EmsID = csCIDPull.EmsID
;

如果删除 INNER JOIN EMSData 部分，它提供 CustomerData 和 csCIDPull 表中的信息。我的思维方式可能不正确。我假设有5个表都带有一个int ID，这些ID还将提交到一个表中以合并所有表（MAIN表仅包含ID，而其他表包含数据）。

This returns NO results, if I remove the INNER JOIN EMSData section, it provides the info from CustomerData and csCIDPull tables. My method of thinking may be incorrect. I have let's say 5 tables all with a int ID, those ID's are also submitting to a single table to combine all tables (the MAIN table contains only ID's while the other tables contain the data).

想通了，我会向你们射击，看看我可能做错了什么。 -谢谢

Figured I'd shoot you folks posting to see what I might be doing wrong. -Thanks

推荐答案

基本上，听起来您的EMSData表中没有匹配的数据。您需要为此使用外部联接：

Basically it sounds like you don't have matching data in your EMSData table. You would need to use an OUTER JOIN for this:

SELECT *
FROM csusaCIDPull
    LEFT JOIN CustomerData ON CustomerData.CustomerID = csCIDPull.CustomerID
    LEFT JOIN EMSData ON EMSData.EmsID = csCIDPull.EmsID

A SQL连接的视觉解释

侧面说明：考虑不返回 * 而是选择每个表格中您想要的字段。

Side note: consider not returning * but rather select the fields you want from each table.

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