SQL 为空且 = 空
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SQL is null and = null
提问by Sachin Kainth
Possible Duplicate:
what is “=null” and “ IS NULL”
Is there any difference between IS NULL and =NULL
What is the difference between
之间有什么区别
where x is null
and
和
where x = null
and why does the latter not work?
为什么后者不起作用？
回答by Bohemian
In SQL, a comparison between a nullvalue and any other value (including another null) using a comparison operator (eg =, !=, <, etc) will result in a null, which is considered as falsefor the purposes of a where clause (strictly speaking, it's "not true", rather than "false", but the effect is the same).
在SQL中，之间的比较null值，任何其他值（包括另一种null使用比较操作符（如）=，!=，<，等）将导致null，这被认为是false一个where子句的目的（严格来说，它的“不true”，而不是“false”，但效果是一样的）。
The reasoning is that a nullmeans "unknown", so the result of any comparison to a nullis also "unknown". So you'll get no hit on rows by coding where my_column = null.
推理是 anull表示“未知”，因此与 a 进行任何比较的结果null也是“未知”。因此，您不会通过编码对行产生任何影响where my_column = null。
SQL provides the special syntax for testing if a column is null, via is nulland is not null, which is a special condition to test for a null(or not a null).
SQL 提供了用于测试列是否为null、通过is null和的特殊语法is not null，这是测试 a null（或不是 a null）的特殊条件。
Here's some SQL showing a variety of conditions and and their effect as per above.
这里有一些 SQL 显示了各种条件及其效果，如上所述。
create table t (x int, y int);
insert into t values (null, null), (null, 1), (1, 1);
select 'x = null' as test , x, y from t where x = null
union all
select 'x != null', x, y from t where x != null
union all
select 'not (x = null)', x, y from t where not (x = null)
union all
select 'x = y', x, y from t where x = y
union all
select 'not (x = y)', x, y from t where not (x = y);
returns only 1 row (as expected):
仅返回 1 行（如预期）：
TEST X Y
x = y 1 1
See this running on SQLFiddle
看到这个在SQLFiddle 上运行
回答by Curt
It's important to note, that NULL doesn't equal NULL.
需要注意的是，NULL 不等于 NULL。
NULLis not a value, and therefore cannot be compared to another value.
NULL不是一个值，因此不能与另一个值进行比较。
where x is nullchecks whether x is a null value.
where x is null检查 x 是否为空值。
where x = nullis checking whether x equals NULL, which will never be true
where x = null正在检查 x 是否等于 NULL，这永远不会是真的
回答by Sarfraz
First is correct way of checking whether a field value is nullwhile later won't work the way you expect it to because nullis special value which does not equal anything, so you can't use equality comparison using =for it.
首先是检查字段值是否正确的方法，null而稍后将无法按照您期望的方式工作，因为null它是不等于任何值的特殊值，因此您不能使用=它来进行相等比较。
So when you need to check if a field value is nullor not, use:
因此，当您需要检查字段值是否null存在时，请使用：
where x is null
instead of:
代替：
where x = null
回答by Vikram
I think that equality is something that can be absolutely determined. The trouble with nullis that it's inherently unknown. Nullcombined with any other value is null- unknown. Asking SQL "Is my value equal to null?" would be unknown every single time, even if the input is null. I think the implementation of IS NULLmakes it clear.
我认为平等是可以绝对确定的。问题null在于它本质上是未知的。Null结合任何其他值是null- 未知。询问 SQL “我的值等于null吗？” 即使输入是null. 我认为 的实施已经IS NULL很清楚了。
