lightdb for postgresql 系列【时间加减处理】

lightdb上我们分别可以用下面的函数对时间进行加法和减法运算
方法一：

select hiredate - interval '5 day'   as hd_minus_5D,
       hiredate + interval '5 day'   as hd_plus_5D,
       hiredate - interval '5 month' as hd_minus_5M,
       hiredate + interval '5 month' as hd_plus_5M,
       hiredate - interval '5 year'  as hd_minus_5Y,
       hiredate + interval '5 year'  as hd_plus_5Y
from emp;

方式二：这个与Oracle的方式一样

select hiredate - 5                  as hd_minus_5D,
       hiredate + 5                  as hd_plus_5D,
       add_months(hiredate, -5)      as hd_minus_5M,
       add_months(hiredate, 5)       as hd_plus_5M,
       add_months(hiredate, -5 * 12) as hd_minus_5Y,
       add_months(hiredate, 5 * 12)  as hd_plus_5Y
from emp;

两种方法分别返回timestamp without time zone和date类型

create table t1 as 
    select hiredate - interval '5 day'   as hd_minus_5D,
       hiredate + interval '5 day'   as hd_plus_5D,
       hiredate - interval '5 month' as hd_minus_5M,
       hiredate + interval '5 month' as hd_plus_5M,
       hiredate - interval '5 year'  as hd_minus_5Y,
       hiredate + interval '5 year'  as hd_plus_5Y
from emp;

create table t2 as 
    select hiredate - 5                  as hd_minus_5D,
       hiredate + 5                  as hd_plus_5D,
       add_months(hiredate, -5)      as hd_minus_5M,
       add_months(hiredate, 5)       as hd_plus_5M,
       add_months(hiredate, -5 * 12) as hd_minus_5Y,
       add_months(hiredate, 5 * 12)  as hd_plus_5Y
from emp;

lightdb@postgres=# \d t1
                             Table "public.t1"
   Column    |            Type             | Collation | Nullable | Default 
-------------+-----------------------------+-----------+----------+---------
 hd_minus_5d | timestamp without time zone |           |          | 
 hd_plus_5d  | timestamp without time zone |           |          | 
 hd_minus_5m | timestamp without time zone |           |          | 
 hd_plus_5m  | timestamp without time zone |           |          | 
 hd_minus_5y | timestamp without time zone |           |          | 
 hd_plus_5y  | timestamp without time zone |           |          | 

lightdb@postgres=# \d t2
                  Table "public.t2"
   Column    | Type | Collation | Nullable | Default 
-------------+------+-----------+----------+---------
 hd_minus_5d | date |           |          | 
 hd_plus_5d  | date |           |          | 
 hd_minus_5m | date |           |          | 
 hd_plus_5m  | date |           |          | 
 hd_minus_5y | date |           |          | 
 hd_plus_5y  | date |           |          |