creating partition in external table in hive

Yes, you have to tell Hive explicitly what is your partition field.

Consider you have a following HDFS directory on which you want to create a external table.

/path/to/dataFile/

Let's say this directory already have data stored(partitioned) department wise as follows:

/path/to/dataFile/dept1
/path/to/dataFile/dept2
/path/to/dataFile/dept3

Each of these directories have bunch of files where each file contains actual comma separated data for fields say name,age,height.

e.g.
    /path/to/dataFile/dept1/file1.txt
    /path/to/dataFile/dept1/file2.txt

Now let's create external table on this:

Step 1. Create external table:

CREATE EXTERNAL TABLE testdb.table1(name string, age int, height int)
PARTITIONED BY (dept string)
ROW FORMAT DELIMITED
STORED AS TEXTFILE
LOCATION '/path/to/dataFile/';

Step 2. Add partitions:

ALTER TABLE testdb.table1 ADD PARTITION (dept='dept1') LOCATION '/path/to/dataFile/dept1';
ALTER TABLE testdb.table1 ADD PARTITION (dept='dept2') LOCATION '/path/to/dataFile/dept2';
ALTER TABLE testdb.table1 ADD PARTITION (dept='dept3') LOCATION '/path/to/dataFile/dept3';

Done, run select query once to verify if data loaded successfully.

1. Set below property

set hive.exec.dynamic.partition=true

set hive.exec.dynamic.partition.mode=nonstrict

2. Create External partitioned table

create external table1 ( name string, age int, height int) location 'path/to/dataFile/in/HDFS';

3. Insert data to partitioned table from source table.

Basically , the process is same. its just that you create external partitioned table and provide HDFS path to table under which it will create and store partition.

Hope this helps.