How do i convert column values in a table to row values using hive

Question

eg: current table

employee_id employee_name   location_1  location_2  location_3
111 Reeta   Delhi
112 Pinky   Chennai Kerala  Null
113 Tinku   Noida   Ranchi  Jaipur
114 Teena   Null
115 Bing    Assam   Assam

Desired table:

employee_id employee_name   Location
111 Delhi
112 Pinky   Chennai
112 Pinky   Kerala
113 Tinku   Noida
113 Tinku   Ranchi
113 Tinku   Jaipur
115 Bing    Assam

1.The Null values on location column should be ignored while transforming column to row in target table 2. the employee_id and name with null value as location shouldnt be brought to target table. 3.When emplyee_id and employee_name as duplicate values only one should be brought to target table

Answer 1

Probably the simplest way is a union all . But that requires scanning the table multiple times. So, instead:

select tn.*
from (select t.employee_id, t.employee_name,
             (case when n.n = 1 then location_1
                   when n.n = 2 then location_2
                   when n.n = 3 then location_3
              end) as location 
      from t cross join
           (select 1 as n union all select 2 union all select 3) n
     ) tn
where location is not null;

Answer 2

A simple option uses union all :

select employee_id, employee_name, location_1 location from mytable where location_1 is not null
union all 
select employee_id, employee_name, location_2 from mytable where location_2 is not null
union all 
select employee_id, employee_name, location_3 from mytable where location_3 is not null

A hive-specific approach using a map and lateral view explode might be more efficient:

select employee_id, employee_name, location
from (
    select 
        employee_id, 
        employee_name, 
        map("location_1", location_1, "location_2", location_2, "location_3", location_3) as mp
    from mytable
) t
lateral view explode(mp) m as locname, location  
where location is not null