Hive SQL basics

Hive execution sequence:

**FROM-->WHERE-->GROUP BY-->HAVING-->SELECT-->ORDER BY**

Writing order:

**SELECT DISTINCT
FROM
JOIN
ON
WHERE
GROUP BY
WITH
HAVING
ORDER BY
LIMIT**

HIVE is a Hadoop-based data warehouse
Hive SQL compared with traditional SQL:

Note: If the table is a partitioned table , the partition field must be restricted in the where condition.

select user_name,piece,pay_amount from user_trade
where dt="2019-04-09" and goods_category='food';

The role of GROUP BY: subtotal

SELECT user_name,sum(pay_amount) AS total_amount
FROM user_trade WHERE dt between '2019-04-01' and '2019-04-30' GROUP BY user_name
HAVING sum(pay_amount)>50000;

HAVING: filter group by objects and only return results that meet the HAVING conditions

How to convert a timestamp to a date?

select pay_time,from_unixtime(pay_time,
'yyyy-MM-dd hh:mm:ss')
from user_trade
where dt='2019-04-09';

format:
1.yyyy-MM-dd hh:mm:ss
2.yyyy-MM-dd hh
3.yyyy-MM-dd hh：mm
4.yyyyMMdd

– How to retrieve users who are in user_list_1 but not in user_list_2?

select a.user_id,
	    a.user_name
from user_list_1 a left join user_list_2 b on a.user_id=b.user_id
where b.user_id is null;

--注：MySQL中的写法(子查询)
select user_id,
		user_name
from user_list_1
where user_id not in(select user_id from user_list_2)

--在2019年购买但是没有退款的用户
select a.user_name
from
(select distinct user_name 
from user_trade 
where year(dt)=2019)a
left join
(select distinct user_name
from user_refund 
where year(dt)=2019)b
on a.user_name=b.user_name
where b.user_name is null;

-- 在2017年、2018年、2019年都有交易的用户
-- 第一种写法
select distinct a.user_name
from trade_2017 a
join trade_2018 b on a.user_name=b.user_name
join trade_2019 c on b.user_name=c.user_name;

-- 第二种写法(在表的数据量很大时，推荐这种写法，hive中建议这种写法)
select a.user_name 
from
	(select distinct user_name 
	from trade_2017)a
join
	(select distinct user_name 
	from trade_2018)b on a.user_name=b.user_name
join
	(select distinct user_name 
	from trade_2019)c on b.user_name=c.user_name;