表

只拷贝表结构，不拷贝表数据
CREATE TABLE ruozedata_emp2 LIKE ruozedata_emp;
show create table xxx(表名）；可看到表结构

拷贝表数据
Create Table As Select (CTAS)
create table ruozedata_emp3 as select empno,ename,deptno from ruozedata_emp;
可解决需求：天粒度，今天统计的是昨天的，统计结果写到一个tmp表里面

改表名，路径的名称也改了
ALTER TABLE 老表 RENAME TO 新表;
ALTER TABLE ruozedata_emp3 rename to ruozedata_emp3_bak;

Drop Table  删表跑路
DROP TABLE  table_name ruozedata_emp3_bak;

Truncate Table  XXX  删表的数据

trash   删除移动到此垃圾桶，需配置

内部表和外部表

metadata： TBL_TYPE
	data：  Table Type    

MANAGED_TABLE    内部表
DROP ： data  +  metadata   X
EXTERNAL_TABLE     外部表
DROP: metadata X      HDSF  √
删除内部表会把数据和元数据都删除，而删除外部表只删除元数据。
从安全各方面出发，生产上使用外部表。

创建外部表
配路径
CREATE EXTERNAL TABLE emp_external(
empno int,
ename string,
job string,
mgr int,
hiredate string,
sal double,
comm double,
deptno int
)ROW FORMAT DELIMITED FIELDS TERMINATED BY '\t'
LOCATION '/d7_externel/emp/' ;

Load加载数据

create table ruozedata_dept(
deptno int,
dname string,
loc string
) ROW FORMAT DELIMITED FIELDS TERMINATED BY '\t';

    load data local inpath '/home/hadoop/data/dept.txt' into table ruozedata_dept;
    
    load data inpath 'hdfs://hadoop000:8020/wc/dept/dept.txt' into table ruozedata_dept;

LOAD DATA [LOCAL] INPATH '' [OVERWRITE] INTO TABLE XXX;
LOCAL：从本地系统  linux服务器
不带LOCAL: 从Hadoop文件系统  HDFS

OVERWRITE           数据覆盖
不带OVERWRITE    追加

从查询结果集导入/导出数据

INSERT OVERWRITE TABLE tablename1 [PARTITION (partcol1=val1, partcol2=val2 ...) [IF NOT EXISTS]] 
select_statement1 FROM from_statement;

INSERT INTO TABLE tablename1 [PARTITION (partcol1=val1, partcol2=val2 ...)] select_statement1 FROM from_statement;

Writing data into the filesystem from queries   导出
INSERT OVERWRITE LOCAL DIRECTORY '/home/hadoop/tmp/empout'
ROW FORMAT DELIMITED FIELDS TERMINATED BY '\t'  
SELECT empno,ename FROM ruozedata_emp;

聚合函数

   count max min sum avg 多进一出
    select max（sal）,min(sal),sum(sal),avg(sal) from xxx_emp

select deptno,avg(sal)  from xxx_emp group by deptno;
出现在select子句中的字段要么出现在group by中要么出现在聚合函数中

    selectXXX，case开头
    when中间 XXX
    end结尾 from XXX_emp;
    生产上案例： 
    select ename,sal,
    case
    when sal>1 and sal<=1000 then "lower"
    when sal>1000 and sal<=2000 then "just so so"
    when sal>2000 and sal<=4000 then "ok"
    else "high"
    end
    from ruozedata_emp;

order by vs sort by vs distribute by vs cluster by

order by 全局排序    1reduce  X不要使用，若使用用limit
sort by 局部排序     每个reduce内是有序

set hive.mapred.mode=nonstrict；非严格模式
set hive.mapred.mode=strict；严格模式
set mapred.reduce.tasks=3;

INSERT OVERWRITE LOCAL DIRECTORY '/home/hadoop/tmp/sortby'
ROW FORMAT DELIMITED FIELDS TERMINATED BY '\t'  
SELECT * FROM ruozedata_emp sort by empno desc;

distribute by

   按照一定的规则把数据分发到某个reducer

    INSERT OVERWRITE LOCAL DIRECTORY '/home/hadoop/tmp/distributeby'
    ROW FORMAT DELIMITED FIELDS TERMINATED BY '\t'  
    SELECT * FROM ruozedata_emp 
    distribute by length(ename) 
    sort by empno ;

cluster by

==> distribute by xxx sort by xxx

INSERT OVERWRITE LOCAL DIRECTORY '/home/hadoop/tmp/clusterby'
ROW FORMAT DELIMITED FIELDS TERMINATED BY '\t'  
select * from ruozedata_emp cluster by ename

Hive内置函数

length(ename)Hive里面的内置函数 build-in
show functions;
desc function length;
desc function extended length; 

Hive支持insert update delete，但是不用
离线计算，用load，一定不要把hive当做RDBMS来用

date function

cast 转换

select cast("5" as int) from dual;
select cast("2019-08-04" as date) from dual;
select cast(current_timestamp as date) from dual;
转不成功null

数字

round（四舍五入）
select round （3.141592,4）from dual；
ceil  /ceiling（往上取整）
floor(往下）
ABS（绝对值）
least（返回最小的值）
select least（3,5,6）from dual;
greatst(返回最大的值)

字符串

desc function extended substr;
select substr("abcdefg",2,3)from dual;
concat(拼起来)
concat_ws(用.点拼接起来)

案例：Hive来完成wc统计

/data/wc.data

hello   hello   hello
world   world
welcome


hive

create table wc(sentence string);
load data local inpath ' /home/hadoop/data/wc.data' into table wc;
select * from wc;
select  explode (split（sentence，'\t')）)from wc;
select  explode (split（sentence，'\t')）)as word from wc;

hello   hello   hello
==> split  explode     一进多出，行转列
hello
hello
hello

非严格模式下

select word,count(1) as cnt
from
(
select explode(split(sentence,'\t')) as word from wc
) t
group by word
order by cnt desc;