Replace the empty or NULL value with specific valu

问题:

I'm trying to show a default value, "Others", when the query does not return any result for one of the selected columns. I'll show you the example.

This query returns an empty value for os(agent) SO (in the first row):

select country, os(agent) SO, count(*) from clicks_data
where country is not null and os(agent) is not null
group   by country, os(agent);

Output:

ZA           4
ZA  Android  4
ZA  Mac      8
ZA  Windows  5

Instead, I would like to get this result:

ZA  Others  4
ZA  Android 4
ZA  Mac     8
ZA  Windows 5

My next attempt was this query, but it's not really working, either:

select country, regexp_replace(os(agent),'','Others') SO, count(*) from clicks_data 
where country is not null and os(agent) is not null 
group by country, os(agent);

This is the result:

ZA  Others  4
ZA  OthersAOthersnOthersdOthersrOthersoOthersiOthersdOthers 4
ZA  OthersMOthersaOtherscOthers 8
ZA  OthersWOthersiOthersnOthersdOthersoOtherswOtherssOthers 5

回答1:

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!!!

回答2:

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.