在SELECT和WHERE子句上使用相同的表

Question

简单案例

我有一个简单的表，其中一列是同一表中另一个元素的PK，如下所示：

    id     |  something   | parent_id
___________|______________|____________
     1     |   colors     |  null
     2     |   red        |  1
     3     |   green      |  1
     4     |   blue       |  1
    ...    |   ...        | ...

任务

我想获取所有具有条件的元素parent_id的元素。 假设父元素应具有something='colors'

预期结果

    id     |  something   | parent_id
___________|______________|____________
     2     |   red        |  1
     3     |   green      |  1
     4     |   blue       |  1

---

环顾StackOverflow之后，我尝试了：

SELECT * FROM my_table WHERE 
   parent_id=(SELECT id FROM my_table WHERE something='colors')

还有：

SELECT * FROM my_table as t1 WHERE
   t1.parent_id=(SELECT id FROM my_table as t2 WHERE t2.something='colors')

但我收到此错误：

  

错误：您的SQL语法有错误

原因是在我的文件之前被合并的先前文件中缺少;。以上查询确实有效 ...

---

这不是我的领域，我知道一个“ 解决方案”（？）（项目中其他人使用的 ）可能是：

SELECT * FROM my_table WHERE 
    parent_id=(SELECT id FROM (SELECT * FROM my_table) AS t1 WHERE something='colors')

但是我不是很喜欢

---

感谢任何人的帮助。 我知道我们应该LIMIT 1或使用IN来防止错误，我正尽最大努力简化sintax。

Answer 1

使用存在

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Answer 2

您可以：

SELECT t.*
FROM table t
WHERE EXISTS(SELECT 1 FROM table t1 WHERE t1.id = t.parent_id AND t1.something = 'colors');

Answer 3

您可以通过简单的JOIN来获得它：

SELECT T1.* FROM my_table AS T1 
JOIN my_table AS T2 ON T1.parent_id = T2.id
WHERE T2.something='colors';

此技术称为“自连接”。 Here了解更多详情。