blocks|key|1581258|text|您应该像这样运行您的sql查询...|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1581259|SELECT+*+from+schools+
WHERE+`district_id`+=+
(SELECT+district_id+from+schools+
++++WHERE+`school_id`+=+'0023194'
++++AND+
++++district_id+IS+NOT+NULL)+
AND+district_id+IS+NOT+NULL|code-block|syntax|javascript|1581260|1581261|+district_id不为空以防止district_id不为空|blockquote|1581262|1581263|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|O|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|P|8|@]|9|@]|A|$]]|$1|H|3|I|5|J|7|Q|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|R|8|@]|9|@]|A|$]]|$1|L|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|M|$]]

you should run your sql query like ...

<pre><code>SELECT * from schools 
WHERE `district_id` = 
(SELECT district_id from schools 
 WHERE `school_id` = '0023194'
 AND 
 district_id IS NOT NULL) 
AND district_id IS NOT NULL
</code></pre>

<blockquote>
 district_id IS NOT NULL
 to prevent district_id not null
</blockquote>

blocks|key|1634416|text|看起来你只需要像下面这样使用IN：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1634417|SELECT+*+from+schools+
WHERE+`district_id`+in
(SELECT+district_id+from+schools+WHERE+`school_id`+=+'0023194')|code-block|syntax|javascript|1634418|SQL+HERE|1634419|entityMap|0|LINK|mutability|MUTABLE|url|http://sqlfiddle.com/#!9/9440a2/2^0|E|2|0|0|0|8|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|Y|8|@]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|M|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]]]

Seems you just need to use <code>IN</code> like below :

<pre><code>SELECT * from schools 
WHERE `district_id` in
(SELECT district_id from schools WHERE `school_id` = '0023194')
</code></pre>

<a href="http://sqlfiddle.com/#!9/9440a2/2" rel="nofollow noreferrer">SQL HERE</a>

blocks|key|5039979|text|可以使用IN运算符，因为IN运算符允许您在WHERE子句中指定多个值。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|5039980|IN运算符是多个OR条件的简写。在您的条件下，=只比较一个条件，但是如果您有此school_id的多个记录，=将无法对其求值。|5039981|SELECT+*+from+schools+WHERE+`district_id`+in+(SELECT+district_id+from+schools+WHERE+`school_id`+=+'0023194')|code-block|syntax|javascript|5039982|entityMap^0|4|2|0|N|1|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|R|8|@$9|S|A|T|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|U|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|N|$]]

You can use <code>IN</code> operator because IN operator allows you to specify multiple values in a WHERE clause. 

IN operator is a shorthand for multiple OR conditions. In your condition, <code>=</code> compares only one condition but if you have multiple record of this school_id, = can't evaluate it.

<pre><code>SELECT * from schools WHERE `district_id` in (SELECT district_id from schools WHERE `school_id` = '0023194')
</code></pre>

I only want to run the parent select if the subselect is not null. Unsure how to achieve this using sql query :-

<pre><code>SELECT * from schools 
 WHERE `district_id` =
 (SELECT district_id from schools WHERE `school_id` = '0023194')
</code></pre>

How to deal with subquery returning null

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我只想在子选择不为空的情况下运行父选择。不确定如何使用sql查询实现这一点：SELECT * from schools     WHERE `district_id` =    (SELECT district_id from schools WHERE `school_id` = '0023194')

问如何处理子查询返回空值
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问如何处理子查询返回空值EN