What is the output of below query?
select * from test1 t1
inner join test2 t2
on t1.ID = t2.ID
where t1.ID <> t2.ID;

It will have no records. As we specify equal on ON clause but specify <> in where clause and alternatively where clause will return true or false based on the conditions given.
So no records will be returned as condition is getting false.If we specify other condition which gets satisfied,then we get the records.