MySql <> NULL和IS NOT NULL之间的区别

Question

In my db there is a varchar(255) column and for some records it contains null values, when i fired this

SELECT * FORM my_table where some_column <> NULL;

nothing is returned

but when is fired

SELECT * FORM my_table where some_column IS NOT NULL;

I got desired records

can you explain what's the main difference between them and when to use <> and != operators.

Answer 1

From the Mysql 8 Reference:

You cannot use arithmetic comparison operators such as =, <, or <> to test for NULL. Because the result of any arithmetic comparison with NULL is also NULL, you cannot obtain any meaningful results from such comparisons.

Answer 2

In mysql a string column can be empty = '' or NUL (not values assigned )

so you should use for not empty

  SELECT * FORM my_table where some_column <> '';

or

  SELECT * FORM my_table where some_column  != '';

or not equal to a value

  SELECT * FORM my_table where some_column <> 'my_value';
  SELECT * FORM my_table where some_column != 'my_value';

for not null you must use the special operator IS NOT NULL

SELECT * FORM my_table where some_column IS NOT NULL;