为什么case语句返回null值

Question

I am looking to group bmi-ranges and the average child per range. Everything seems to check out but i am recieve a null group when there are no nulls in the data set. This is the code:

    With new_table as 
(
    SELECT 
    case 
    when bmi <15 THEN 'Under 15'
    when bmi BETWEEN 15 AND 20 THEN '20 and under'
    when bmi BETWEEN 20 AND 29.99 THEN '20-29'
    when bmi BETWEEN 30 AND 39.99 THEN '30-39'
    when bmi BETWEEN 40 AND 49.99 THEN '40-49'
    when bmi >50 THEN '50 and over'

    END as bmi_range,
    AVG(children) as children
FROM 
    health_in.data_in 
WHERE 
    children != 0
GROUP BY    
    case 
    when bmi <15 THEN 'Under 15'
    when bmi BETWEEN 15 AND 20 THEN '20 and under'
    when bmi BETWEEN 20 AND 29.99 THEN '20-29'
    when bmi BETWEEN 30 AND 39.99 THEN '30-39'
    when bmi BETWEEN 40 AND 49.99 THEN '40-49'
    when bmi >50 THEN '50 and over'
    END
)
SELECT  
    *
FROM 
    new_table 
GROUP BY 
    bmi_range,
    children
ORDER BY 
    bmi_range asc

there are no rows with in the source table health_in.data_in with a null bmi (or with any value less than 15 or more than 60).

Why does bmi_range then include null values?

Answer 1

The CASE does not cover all possible values, eg 50, or 29.995. Also there is no need to repeat the values because a CASE expression uses short-circuit evaluation, ie it stops as soon as a match is found.

You can rewrite it as:

CASE 
WHEN bmi < 15 THEN 'Under 15'
WHEN bmi < 20 THEN '15-20'
WHEN bmi < 30 THEN '20-29'
WHEN bmi < 40 THEN '30-39'
WHEN bmi < 50 THEN '40-49'
ELSE '50 and over'
END