SQL - 内部联接

Question

我的查询如下：

SELECT t1.onderzoeksNR, t1.antwoorden as b1_m1_v1_items, t1.commentaar as b1_m1_v1_toe, t2.antwoorden as b1_m1_v2_items, t2.commentaar as b1_m1_v2_toe
FROM antwoorden t1
INNER JOIN antwoorden t2
ON t1.onderzoeksNR = t2.onderzoeksNR
WHERE t1.behandeltraject = 1 AND t1.meting = 1 AND t1.vragenlijst = 1
AND t2.behandeltraject = 1 AND t2.meting = 1 AND t2.vragenlijst = 2

这将输出的是：

这正是我想要的。

然而，问题是：

如果我添加另一个内连接

inner join antwoorden t3 
on t1.onderzoeksNR = t3.onderzoeksNR

但是喜欢：

t3.behandeltraject = 1 AND t3.meting = 2 AND t3.vragenlijst = 1

查询将返回0条记录。

显然，这是因为antwoorden中包含所询问属性的无行。

（t3.behandeltraject = 1 AND t3.meting = 2 AND t3.vragenlijst = 1）

我的问题：

1. 我怎样才能检索开头的内容，

2. 然后将内部联接antwoorden t3添加到其中，但之后添加空值（如果找不到t3.behandeltraject = 1 AND t3.meting = 2 AND t3.vragenlijst = 1的行）？

说明我的意思：



而不是返回0条记录。

非常感谢任何帮助

Answer 1

您需要LEFT JOIN，并且需要将一些WHERE条件移至JOIN条件：

SELECT t1.onderzoeksNR, t1.antwoorden as b1_m1_v1_items, t1.commentaar as b1_m1_v1_toe, t2.antwoorden as b1_m1_v2_items, t2.commentaar as b1_m1_v2_toe
FROM antwoorden t1
INNER JOIN antwoorden t2
ON t1.onderzoeksNR = t2.onderzoeksNR
LEFT JOIN antwoorden t3 ON t1.onderzoeksNR = t3.onderzoeksNR
     AND t3.behandeltraject = 1 AND t3.meting = 2 AND t3.vragenlijst = 1
WHERE t1.behandeltraject = 1 AND t1.meting = 1 AND t1.vragenlijst = 1
AND t2.behandeltraject = 1 AND t2.meting = 1 AND t2.vragenlijst = 2

LEFT JOIN将允许前两个表中的记录返回，无论他们是JOIN到t3，还是使用WHERE条件过滤记录{ {1}}等等，您将删除结果。将t3.meting = 2条件移至WHERE将返回与初始查询相同的记录，仅添加JOIN的{​​{1}}，因为没有符合条件。

Answer 2

您应该在最后一个中使用LEFT OUTER JOIN而不是INNER JOIN：

LEFT JOIN antwoorden t3 ON t1.onderzoeksNR = t3.onderzoeksNR