How to retrieve null values and null values in a database

The database does not have the following values:

1. null if it seems that the cell is blank, It is the null value.
2.0 occasionally, some cells are '0' by default. The value 0 may be char or Int, so it is divided into 0 and '0' (that is, single quotes + 0 ).
3. null This NULL is not another null, And This NULL is 'null' (that is, single quotes + null). I have met it once, inCodeAll null values are filled with null. It is intended to be filled with null, and the result is eventually 'null', which is a string consisting of four letters.
To retrieve all the above three items, you can use the following sentence, even if you do not match all the three points, you can use it. It is good to skip one.
Select id_gradebase, id_college, id_majorsecmajor
From xsstu
Where id_gradebase = '0' or id_college = '0' or id_majorsecmajor = '0' or isnull (id_gradebase, '') +'' = ''or isnull (id_college ,'') + ''='' or isnull (id_majorsecmajor, '') +'' = ''or id_gradebase = 'null' or id_college = 'null' or id_majorsecmajor = 'null'