SQL> select empno from emp where 1=2;

no rows selected

SQL> select max(empno) from emp where 1=2;
MAX(EMPNO)
----------


1 row selected.

I run it and can interpret result, since 1<>2, query will return no rows.

Why the output shows 'No rows selected' in first case and '1 row seelected' when we used max(empno)?

All aggregate functions except COUNT(*) and GROUPING ignore nulls. You can use the NVL function in the argument to an aggregate function to substitute a value for a null. COUNT never returns null, but returns either a number or zero. For all the remaining aggregate functions, if the data set contains no rows, or contains only rows with nulls as arguments to the aggregate function, then the function returns null.

Also where can such logic( where 1=2 or where 1=1) be applied in any application ?

Quote:

Also where can such logic( where 1=2 or where 1=1) be applied in any application ?

Bad programming.

if input_parameter = 'Y' then
   lex_parameter := 'and cb_active = 1';
else
   lex_parameter := 'and 1 = 1';
end if;

select id, ename, address
from some_table
where hiredate >= trunc(sysdate, 'yyyy')
  &lex_parameter

where hiredate >= trunc(sysdate, 'yyyy')
  and cb_active = 1

where hiredate >= trunc(sysdate, 'yyyy')
  and 1 = 1