这个操作符是什么<=>?在 MySQL 中?

2021-11-20 00:00:00 sql operators mysql spaceship-operator

我正在处理由以前的开发人员编写的代码,并且在查询中说,

I'm working on code written by a previous developer and in a query it says,

WHERE p.name <=> NULL

<=> 在这个查询中是什么意思?它等于 = 吗?还是语法错误?

What does <=> mean in this query? Is it something equal to =? Or is it a syntax error?

但它没有显示任何错误或异常.我已经知道 <> = != 在 MySQL.

But it is not showing any errors or exceptions. I already know that <> = != in MySQL.

推荐答案

TL;DR

这是 NULL安全等于运算符.

TL;DR

It's the NULL safe equal operator.

与常规的=运算符一样,比较两个值,结果是0(不相等)或1(相等);换句话说:'a' <=>'b' 产生 0'a' <=>'a' 产生 1.

Like the regular = operator, two values are compared and the result is either 0 (not equal) or 1 (equal); in other words: 'a' <=> 'b' yields 0 and 'a' <=> 'a' yields 1.

与常规的 = 运算符不同,NULL 的值没有特殊含义,因此它永远不会产生 NULL 作为可能的结果;所以:'a' <=>NULL 产生 0NULL <=>NULL 产生 1.

Unlike the regular = operator, values of NULL don't have a special meaning and so it never yields NULL as a possible outcome; so: 'a' <=> NULL yields 0 and NULL <=> NULL yields 1.

当两个操作数都可能包含 NULL 并且您需要两列之间的比较结果一致时,这会很有用.

This can come in useful when both operands may contain NULL and you need a consistent comparison result between two columns.

另一个用例是准备好的语句,例如:

Another use-case is with prepared statements, for example:

... WHERE col_a <=> ? ...

此处,占位符可以是标量值或 NULL,而无需更改查询的任何内容.

Here, the placeholder can be either a scalar value or NULL without having to change anything about the query.

除了<=>还有另外两个运算符可以用来与NULL进行比较,即IS NULL和<代码>不为空;它们是 ANSI 标准的一部分,因此在其他数据库上受支持,这与 <=> 不同,后者是 MySQL 特定的.

Besides <=> there are also two other operators that can be used to compare against NULL, namely IS NULL and IS NOT NULL; they're part of the ANSI standard and therefore supported on other databases, unlike <=>, which is MySQL-specific.

您可以将它们视为 MySQL <=> 的特化:

You can think of them as specialisations of MySQL's <=>:

'a' IS NULL     ==> 'a' <=> NULL
'a' IS NOT NULL ==> NOT('a' <=> NULL)

基于此,您的特定查询(片段)可以转换为更便携:

Based on this, your particular query (fragment) can be converted to the more portable:

WHERE p.name IS NULL

支持

SQL:2003 标准为此引入了一个谓词,其工作方式与 MySQL 的 <=> 操作符完全相同,形式如下:

Support

The SQL:2003 standard introduced a predicate for this, which works exactly like MySQL's <=> operator, in the following form:

IS [NOT] DISTINCT FROM 

以下内容得到普遍支持,但相对复杂:

The following is universally supported, but is relative complex:

CASE WHEN (a = b) or (a IS NULL AND b IS NULL)
     THEN 1
     ELSE 0
END = 1

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