16-Hive DDL DML built-in functions

Create a table: the following format

CREATE [TEMPORARY] [EXTERNAL] TABLE [IF NOT EXISTS] [db_name.] table_name
LIKE existing_table_or_view_name
[LOCATION hdfs_path];

Copy table structure, do not copy data table
CREATE TABLE ruozedata_emp2 LIKE ruozedata_emp;

Create Table As Select (CTAS) #CTAS表示Create Table As Select
create table ruozedata_emp3 as select empno,ename,deptno from ruozedata_emp;

Offline Job: day granularity, today is yesterday's statistics, statistics written inside a tmp table

ALTER TABLE 老表 RENAME TO 新表;
ALTER TABLE ruozedata_emp3 rename to ruozedata_emp3_bak;

Drop Table Delete Table foot
DROP TABLE table_name ruozedata_emp3_bak; tables and data will be deleted

Data Truncate Table delete the table, but the table still
Truncate Table table_name;

Drop Table and frequently asked interview Truncate Table

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内部表和外部表
metadata： TBL_TYPE
data： Table Type

hive中有2大类，managed table 和 external table，如下 #external是外部的意思，外部表时候，通常把数据不放在默认的路径下

默认情况下，create table table_name就是内部表

create external table table_name 是外部表

上面的发现在mysql中有元数据，在hdfs中也有数据

所以对于managed内部表而言，执行drop table table_name时候，元数据和HDFS上面的数据都没了

下面来看外部表 terminated,外部表，公司一般走路径，也就是说不放在默认的路径中

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/' ;

这个时候元数据和hdfs中都有数据了

但是HDFS中数据还在

总结:

MANAGED_TABLE
DROP ： data + metadata 都会被删除掉

EXTERNAL_TABLE
DROP: metadata 被删除但是 HDSF不删

上面的2个区别，面试经常会被问到

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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;

上面数据就重复了，此时需要用overwrite 防止重写

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
不带LOCAL: 从Hadoop文件系统 HDFS

OVERWRITE 数据覆盖
不带OVERWRITE 追加

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下面来把数据导出

INSERT OVERWRITE LOCAL DIRECTORY '/home/hadoop/tmp/empout'
ROW FORMAT DELIMITED FIELDS TERMINATED BY '\t'
SELECT empno,ename FROM ruozedata_emp;

SELECT empno,ename FROM ruozedata_emp;

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; #显示如下

16-Hive DDL DML built-in functions

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