MySQL IS NOT NULL和！=之间的区别 [英] Difference between MySQL IS NOT NULL and != ''

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MySQL之间有什么区别

Is there any difference between MySQL

IF (myText IS NOT NULL) THEN

和

IF (myText != '') THEN

是的，

Yes there is a big difference between a NULL value and a blank/empty value.

myText IS NULL时:

myText IS NOT NULL evaluates to FALSE
myText != '' evaluates to NULL (which essentially behaves the same as FALSE would in this specific case you wrote)

但是，您不应该养成对它们一视同仁的习惯，因为在大多数情况下，它们的行为方式会有所不同:例如:

However, you should not get into the habit of treating them the same, since most of the time they will behave differently: For example:

假设您有一个表tbl:

id   text
1    NULL
2    
3    abc

注意:1包含NULL值，2包含空字符串('').

Note: 1 contains a NULL value, and 2 contains an empty string ('').

如果您运行以下查询:

SELECT * FROM tbl WHERE text != ''

...它将返回记录3.

... it will return record 3.

如果您运行以下查询:

SELECT * FROM tbl WHERE text IS NOT NULL

...它将返回记录2和3.

... it will return records 2 and 3.

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MySQL IS NOT NULL和！=之间的区别 [英] Difference between MySQL IS NOT NULL and != &#39;&#39;