Efficient query for average count in SQL

Question

I have a table:

id age tag 
1  26  green
1  26  blue
2  28  yellow
3  45  red
4  40  blue
4  40  green
5  50  red

I need to find both the average and standard deviation of the number of distinct tags per age group (I have coded this with CASE WHEN).

So to get for example:

average_age average_tags sd_tags age_group
26.7        1.5          0.1     25-34
40          2            0       35-44
45          1            0.01    44+  

Is there a way to more efficiently calculate the average count of tags and also to avoid repeating the CASE code in the sub-query for efficiency / readability?

SELECT  
     AVG(age) as average_age,
    AVG(countField) as average_spread,
    STDDEV(countField) as sd_spread,
    CASE
            WHEN v.age BETWEEN 25 AND 34 THEN '25-34'
    
            WHEN v.age BETWEEN 35 AND 44 THEN '35-44'
    
            ELSE '44+' 
        END AS age_group,
    FROM  vid v
    
         (
         SELECT AVG(v.age),
          COUNT(DISTINCT(p.tag)) AS countField,
          CASE
    
            WHEN v.age BETWEEN 25 AND 34 THEN '25-34'
    
            WHEN v.age BETWEEN 35 AND 44 THEN '35-44'
    
            ELSE '44+' 
    
        END AS age_group,
          FROM  vid v
    
           GROUP BY age_group
           ) as q WHERE age IS NOT NULL 
           GROUP BY age_group ORDER BY age_group;

Answer 1

you can use this query

select AVG(T.Age) as average_age,AVG(countField) as average_spread,age_group 
            from (select v.Age,COUNT(DISTINCT(v.Tag)) as countField,CASE    
            WHEN v.age BETWEEN 25 AND 34 THEN '25-34'
            WHEN v.age BETWEEN 35 AND 44 THEN '35-44'
            ELSE '44+' END AS age_group 
            from VID v
            group by v.Age) T
            WHERE age IS NOT NULL 
            GROUP BY age_group ORDER BY age_group;