Postgresql查询七天内数据

查询周月年

SELECT
	* 
FROM
	system_login_info 
WHERE
	date BETWEEN ( SELECT now( ) - INTERVAL '7 day' ) 
	AND ( SELECT now( ) )
        
--可用来替换between后的起始值
--近一周
  select now() - interval '1 week';

--近一月
 select now() - interval '1 month';

--近一年
 select now() - interval '1 year';

--下一周
  select now() + interval '1 week';

--下一月
 select now() + interval '1 month';

--下一年
 select now() + interval '1 year';

查询当前周月年

SELECT * FROM (
--当前时间
SELECT 'now' as type, CURRENT_DATE as time
UNION ALL
--本周开始时间
SELECT 'week' as type, CURRENT_DATE - (extract(DOW from CURRENT_DATE) || ' day')::interval as time
UNION ALL
--本月开始时间
SELECT 'month' as type, to_date(to_char(now(),'yyyy-mm-01'),'yyyy-mm-dd') as time
UNION ALL
--本年开始时间
SELECT 'year' as type, to_date(to_char(now(),'yyyy-01-01'),'yyyy-mm-dd') as time 
)

这个没看懂，有大神解释下吗