postgres統計妹半個小時的數據

SELECT overlay(
to_char(c_sys_date,'yyyy-mm-dd hh24:mi')
placing (
CASE WHEN CAST((cast(substr(CAST ((date_part('min',c_sys_date))/30 AS text), 1,1)as integer)*30) as text) = '30'
THEN CAST((cast(substr(CAST ((date_part('min',c_sys_date))/30 AS text), 1,1)as integer)*30) as text)
ELSE '00'
END )
from 15 for 2)as ia,
count(c_id) from h_user 
GROUP BY ia
ORDER BY ia