See attached. This is a small sample of a table (named "customer") that I have.
Notice the column called cutype in most cases the value is NULL (in italics), I don't know what that means.
I run this query: DELETE from customer where cutype <> 'i'
It deletes the 79 rows that have blank (not NULL) in the column.
It leaves all the rest.
Actually, at this moment, there are NO rows in the table where cutype = 'i'.
Why does it leave the NULL ones? That is not a value of 'i'.