PostgreSQL date handler usage

postgresql Date handler usage

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select now(); -- current time  2021-04-21 11:36:56

SELECT EXTRACT (year FROM now()); -- year  2021

SELECT EXTRACT (quarter FROM now()); -- season  02

SELECT EXTRACT (month FROM now()); -- month  04

SELECT EXTRACT (day FROM now()); -- In the middle of the moon  21

SELECT EXTRACT (week FROM now()); -- Zhou  16

SELECT EXTRACT (isodow FROM now()); -- Zhou Zhongtian  3

SELECT EXTRACT (hour FROM now()); -- When  11

SELECT EXTRACT (minute FROM now()); -- branch  36

SELECT EXTRACT (second FROM now()); -- second  56.909161

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Get year ：extract(year from date )
Get month ：extract(month from date )
Get Quarterly ：extract(quarter from date )

select
 months  month ,max( The highest temperature ) Maximum temperature , min( Minimum temperature ) Minimum temperature , max( Temperature difference )  Temperature difference 
from (
  select (extract(year from  date )||'-'||extract(month from  date )) as  months,  The highest temperature ,  Minimum temperature ,  The highest temperature - Minimum temperature   Temperature difference  
	from weather
  where extract(year from  date ) = '2018'
  ) a group by a.months order by months

postgresql Test data Alias should be added as
mysql Want to follow from dual
postgresql Then don't follow dual

select 2 as id,' technology ' as name, 2000 as value, '' as parent 
union
select 1 as id,' sales ' as name, 5000 as value, '' as parent 
union
select 3 as id,' Telemarketing ' as name, 2000 as value, '1' as parent 
union
select 4 as id,' Store sales ' as name, 3000 as value, '1' as parent 
union
select 5 as id,' Sales promotion ' as name, 2000 as value, '4' as parent 

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select 
' Wuhou District ' as name, 123 as value
union
select
' Qingyang District ' as name, 223 as value
union
select
' Jinniu District ' as name, 453 as value
union
select
' Shuangliu district ' as name, 423 as value
union
select
' Xindu District ' as name, 163 as value
union
select
' Pidu district ' as name, 193 as value
union
select
' Wenjiang District ' as name, 343 as value
union
select
' Qingbaijiang District ' as name, 443 as value
union
select
' Jinjiang District ' as name, 403 as value
union
select
' Chenghua District ' as name, 553 as value
union
select
' Shuangliu district ' as name, 553 as value
union
select
' Chengdu ' as name, 489 as value
union
select
' Sichuan Province ' as name, 500 as value
union
select
' Guangdong province, ' as name, 200 as value
union
select
' Xinjiang Uygur Autonomous Region ' as name, 1000 as value
union
select
' Qinghai Province ' as name, 900 as value
union
select
' Tibet Autonomous Region ' as name, 900 as value

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