Replace the empty or NULL value with specific value in HIVE query result

Use LENGTH() to check the length of the column value. It returns > 0, if there is some value else return 0 for empty or NULL value.

Also frame the column value in CASE WHEN ... END block

The final query may look like:

SELECT country, CASE WHEN LENGTH(os(agent)) > 0 THEN os(agent) ELSE 'Others' END AS SO, COUNT(*) 
FROM clicks_data 
WHERE country IS NOT NULL AND os(agent) IS NOT NULL 
GROUP BY country, os(agent);

Hope this help you!!!

COALESCE will be the best suitable and optimum solution for your case

Syntax: COALESCE(VALUE,DEFAULT_VALUE): Function returns default value when values is null else VALUE;

Query

SELECT country, COALESCE(os(agent),'Others') AS SO, COUNT(*) 
FROM clicks_data 
WHERE country IS NOT NULL AND os(agent) IS NOT NULL 
GROUP BY country, os(agent);

Hope this would be the efficient solution for your problem.