MariaDB 无法评估 SELECT 1 WHERE NULL = NULL; [英] MariaDB cannot evaluate SELECT 1 WHERE NULL = NULL;

问题描述

我目前正在学习 SQL.我有一个我想理解的书中的例子，但 MariaDB 无法评估它:

I'm currently in process of learning SQL. I have an example from a book that I'm trying to understand, but MariaDB cannot evaluate it:

SELECT 1
WHERE NULL = NULL
UNION
SELECT 0
WHERE NULL <> NULL;

然而，这会产生以下错误:

This however gives the following error:

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'WHERE NULL = NULL
UNION
SELECT 0
WHERE NULL <> NULL;' at line 2

我试过了:

SELECT 1;

有效.然而这不是:

SELECT 1 WHERE NULL = NULL;

我也尝试过添加括号，但这并没有太大帮助.我在这里做错了什么?这个例子是为了表明将 NULL 与任何其他值进行比较会得到未知".

I have also tried adding parantheses, but that didn't help much. What am I doing wrong here? This example was to show that comparing NULL with any other value gives "Unknown".

推荐答案

AFAIK 你不能在没有 FROM 的情况下使用 WHERE.SELECT 1 WHERE 1 = 1 也不起作用.

AFAIK you can not use WHERE without FROM. SELECT 1 WHERE 1 = 1 will also not work.

要修复您的示例，您可以从虚拟表中进行选择:

To fix your example you could select from a dummy table:

SELECT 1
FROM (SELECT 'dummy_value') dummy_table
WHERE NULL = NULL
UNION
SELECT 0
FROM (SELECT 'dummy_value') dummy_table
WHERE NULL <> NULL

但了解 MySQL 行为方式的更好方法可能是

But a better way to see how MySQL behaves could be

SELECT 
    NULL = NULL,
    NULL <> NULL,
    NULL IS NULL,
    NULL IS NOT NULL,
    1 = NULL,
    1 <> NULL,
    1 IS NULL,
    1 IS NOT NULL

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