Support aggregate functions in select without from clause, here are some examples as following:
SELECT 1,
'a',
COUNT(),
SUM(1) + 1,
AVG(2) / COUNT(),
MAX(3),
MIN(4),
RANK() OVER() AS w_rank,
DENSE_RANK() OVER() AS w_dense_rank,
ROW_NUMBER() OVER() AS w_row_number,
SUM(5) OVER() AS w_sum,
AVG(6) OVER() AS w_avg,
COUNT() OVER() AS w_count,
MAX(7) OVER() AS w_max,
MIN(8) OVER() AS w_min;
