PostgreSQL日期时间特殊值

记录一些PostgreSQL日期时间特殊值：

select now();
select today;--ERROR: column "today" does not exist
-- 当做常量需要放在单引号中
select 'now'::TIMESTAMP;
select 'today'::TIMESTAMP;
select 'tomorrow'::TIMESTAMP;
select 'yesterday'::TIMESTAMP;