The different between NULL and ''

We can insert NULL into a column of a table.
There are two examples,let's see the difference.
1、INSERT INTO table_name VALUES( id,NULL);
This method inserts NULL to a column of the table.
2、INSERT INTO table_name VALUES( id,'');
This method doesn't insert NULL into the column ofthe table,
because '' is a valid value,though it is a empty string.

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