then perform this select on table 1 (select one),

SELECT IF (1=1,"YES", "NO"), but, when I replace 1=1 with my select, like this:

SELECT IF ((SELECT count(*) FROM Table1)>0,"YES", "NO")

SELECT
    IF(inner_table.count_column > 0, "YES", "NO)  -- here we select the 'count_column' from 'inner_table
FROM
(
    (SELECT count(*) as count_column FROM table1)
) as inner_table -- aliasing the inner select as 'inner_table'