MySQL查询与条件语句? [英] MySQL query with conditional statement?

问题描述

我有一个MySQL查询，保留了两个表的连接.这是当前的查询结果:

I have a MySQL query left joining two tables. This is the current query result:

id | login | privacy-weight | requires
--------------------------------------------
0  | user  | 1              | NULL
0  | user2 | 1              | NULL
0  | user3 | 1              | privacy-weight

查询本身并不重要，因为我只想向查询中添加一个WHERE条件.

The query itself is not important, as I'd only like to add a WHERE condition to the query as it is now.

我只需要提取(用我自己的话)的值:

I need to fetch only values which (in my own words):

IF (`requires` = 'privacy-weight'), then `privacy-weight` must equal = 0;

这意味着，我需要这种情况:

That means, I need this condition:

WHERE `privacy-weight` = 0

但只有在满足以下条件的情况下:

BUT only if this is true:

requires = 'privacy-weight'

可以做到吗?

编辑

显然，这太难理解了，因此，示例输出:

Obviously this is too difficult to understand, therefore, an example output:

privacy-weight | requires
-------------------------
0              | NULL
1              | NULL
0              | privacy-weight
1              | NULL

这些将被忽略(不提取):

These would be ignored (not fetched):

privacy-weight | requires
-------------------------
1              | privacy-weight

推荐答案

您可以使用以下简单测试:

You could use this simple test:

WHERE (requires = 'privacy-weight' AND privacy-weight = 0) OR requires <> 'privacy-weight' OR requires IS NULL

第一部分(requires = 'privacy-weight' AND privacy-weight = 0)阻止输出:

privacy-weight | requires
-------------------------
1              | privacy-weight

但保持:

privacy-weight | requires
-------------------------
0              | privacy-weight

第二部分OR requires <> 'privacy-weight' OR requires IS NULL将保留以下内容:

While the second part OR requires <> 'privacy-weight' OR requires IS NULL will keep the following ones:

privacy-weight | requires
-------------------------
0              | NULL
1              | NULL
1              | NULL

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