A related question about mysqlNULL

For a question about mysqlNULL, I set the filed1 of the database table to not be empty, and set the filed2 field to null & nbsp; and then insert a piece of data, filed1 = 'test' filed2 remains unchanged. then I write a query statement selct * fromtablewherefile about mysql NULL.
I set the filed1 and filed2 fields of the database table to be empty.

Insert a piece of data, and filed1 = 'test' filed2 remains unchanged.

Then I write a query statement.
Selct * from table where filed2 = ''why cannot It be queried?
I changed filed2 to NULL or not.

At last, I set the filed2 structure not to be empty and I can find out what is going on?

------ Solution --------------------
Filed2 is NULL?