[英]Difference between MySQL IS NOT NULL and != ''
Is there any difference between MySQL MySQL之间有什么区别吗?
IF (myText IS NOT NULL) THEN
and 和
IF (myText != '') THEN
Yes there is a big difference between a NULL
value and a blank/empty value. 是的,
NULL
值和空白/空值之间存在很大差异。
Here's one resource that describes the differences. 这是描述差异的一种资源 。
When myText IS NULL
: 当
myText IS NULL
:
myText IS NOT NULL
evaluates to FALSE
myText IS NOT NULL
计算结果为FALSE
myText != ''
evaluates to NULL
(which essentially behaves the same as FALSE
would in this specific case you wrote) myText != ''
求值为NULL
(在你编写的这个特定情况下,它的行为与FALSE
基本相同) However, you should not get into the habit of treating them the same, since most of the time they will behave differently: For example: 但是,你不应该养成对待它们的习惯,因为大多数时候它们会表现不同:例如:
Assume you have a table tbl
: 假设你有一个表
tbl
:
id text
1 NULL
2
3 abc
Note: 1 contains a NULL
value, and 2 contains an empty string ( ''
). 注意: 1包含
NULL
值,2包含空字符串( ''
)。
If you run the following query: 如果您运行以下查询:
SELECT * FROM tbl WHERE text != ''
... it will return record 3. ......它将返回记录3。
If you run the following query: 如果您运行以下查询:
SELECT * FROM tbl WHERE text IS NOT NULL
... it will return records 2 and 3. ...它将返回记录2和3。
Yes there is a difference. 是,有一点不同。
In simple words, myText IS NOT NULL specifies that myText is having some value which could be '' too . 简单来说, myText IS NOT NULL指定myText具有某些可能也是“值”的值 。
Where as myText != '' specifies that it returns TRUE, if myText does NOT contain an empty string . 其中myText!=''指定它返回TRUE,如果myText不包含空字符串 。
There is a difference. 它们是有区别的。 If the default value of a column is "NULL", then if no data has been set for a field, it is truly null.
如果列的默认值为“NULL”,那么如果没有为字段设置数据,则它实际上为空。 However, if the value of a field has been updated as '', it is not NULL, rather it is empty.
但是,如果字段的值已更新为'',则它不是NULL,而是为空。
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