Create, use, and drop an external table

You use an external table, which is a table that Hive does not manage, to import data from a file on a file system, into Hive. In contrast to the Hive managed table, an external table keeps its data outside the Hive metastore. Hive metastore stores only the schema metadata of the external table. Hive does not manage, or restrict access, to the actual external data.

In this task, you need access to HDFS to put a comma-separated values (CSV) file on HDFS. If you do not use Ranger and an ACL is not in place that allows you to access HDFS, you need to log in to a node on your cluster as the hdfs user. Alternatively, when using Ranger, you need to be authorized by a policy, such as the default HDFS all-path policy (shown below) to access HDFS.

In this task, you create an external table from CSV (comma-separated values) data stored on the file system, depicted in the diagram below. Next, you want Hive to manage and store the actual data in the metastore. You create a managed table.

You insert the external table data into the managed table.

This task demonstrates the following Hive principles:

Specifying a database location in the CREATE DATABASE command, for example CREATE DATABASE <managed table db name> LOCATION '<path>' works for managed tables only. To specify the location of an external table, you need to include the specification in the table creation statement as follows:
```
CREATE EXTERNAL TABLE my_external_table (a string, b string)  
                        ROW FORMAT SERDE 'com.mytables.MySerDe' 
                        WITH SERDEPROPERTIES ( "input.regex" = "*.csv")
                        LOCATION '/user/data';       
```
The LOCATION clause in the CREATE TABLE specifies the location of external (not managed) table data.
A major difference between an external and a managed (internal) table: the persistence of table data on the files system after a DROP TABLE statement.
- External table drop: Hive drops only the metadata, consisting mainly of the schema.
- Managed table drop: Hive deletes the data and the metadata stored in the Hive warehouse.

After dropping an external table, the data is not gone. To retrieve it, you issue another CREATE EXTERNAL TABLE statement to load the data from the file system.

Create a text file named students.csv that contains the following lines.

1,jane,doe,senior,mathematics
2,john,smith,junior,engineering

As root, move the file to /home/hdfs on a node in your cluster. As hdfs, create a directory on HDFS in the user directory called andrena that allows access by all, and put students.csv in the directory.
- On the command-line of a node on your cluster, enter the following commands:
```
sudo su -
mv students.csv /home/hdfs
sudo su - hdfs
hdfs dfs -mkdir /user/andrena
hdfs dfs -chmod 777 /user/andrena
hdfs dfs -put /home/hdfs/students.csv /user/andrena
hdfs dfs -chmod 777 /user/andrena/students.csv
```
- Having authorization to HDFS through a Ranger policy, use the command line or Ambari to create the directory and put the students.csv file in the directory.
Start the Hive shell.
For example, substitute the URI of your HiveServer: beeline -u jdbc:hive2://myhiveserver.com:10000 -n hive -p

Create an external table schema definition that specifies the text format, loads data from students.csv located in /user/andrena.

CREATE EXTERNAL TABLE IF NOT EXISTS names_text(
  student_ID INT, FirstName STRING, LastName STRING,    
  year STRING, Major STRING)
  COMMENT 'Student Names'
  ROW FORMAT DELIMITED
  FIELDS TERMINATED BY ','
  STORED AS TEXTFILE
  LOCATION '/user/andrena';

Verify that the Hive warehouse stores the student names in the external table.
SELECT * FROM names_text;

Create the schema for the managed table to store the data in Hive metastore.

CREATE TABLE IF NOT EXISTS Names(
  student_ID INT, FirstName STRING, LastName STRING,    
  year STRING, Major STRING)
  COMMENT 'Student Names';

Move the external table data to the managed table.
INSERT OVERWRITE TABLE Names SELECT * FROM names_text;
Verify that the data now resides in the managed table also, drop the external table metadata, and verify that the data still resides in the managed table.
```
SELECT * from Names; 
DROP TABLE names_text;
SELECT * from Names;                  
```
The results from the managed table Names appears.
Verify that the external table schema definition is lost.
SELECT * from names_text;

Selecting all from names_text returns no results because the external table schema is lost.
Check that the students.csv file on HDFS remains intact.