我正在尝试按年龄段计算人数，几乎可以解决两个问题：

1. 如果没有给定年龄范围（NULL）中的人，则该年龄范围不会出现在结果中。例如，在我的数据中没有“超过80”的条目，因此不会出现日期范围。基本上，当缺少日期范围时，在编程中看起来像是一个错误。

2. 我想以特定方式订购结果。在下面的查询中，由于ORDER BY按age_range，因此‘20-29’的结果位于’Under 20’的结果之前。

这是数据库表“查询”的示例：

inquiry_id  birth_date
1           1960-02-01
2           1962-03-04
3           1970-03-08
4           1980-03-02
5           1990-02-08

这是查询：

SELECT
    CASE
        WHEN age < 20 THEN 'Under 20'
        WHEN age BETWEEN 20 and 29 THEN '20 - 29'
        WHEN age BETWEEN 30 and 39 THEN '30 - 39'
        WHEN age BETWEEN 40 and 49 THEN '40 - 49'
        WHEN age BETWEEN 50 and 59 THEN '50 - 59'
        WHEN age BETWEEN 60 and 69 THEN '60 - 69'
        WHEN age BETWEEN 70 and 79 THEN '70 - 79'
        WHEN age >= 80 THEN 'Over 80'
        WHEN age IS NULL THEN 'Not Filled In (NULL)'
    END as age_range,
    COUNT(*) AS count

    FROM (SELECT TIMESTAMPDIFF(YEAR, birth_date, CURDATE()) AS age FROM inquiries) as derived

    GROUP BY age_range

    ORDER BY age_range

这是基于Wrikken的建议的简单解决方案：

SELECT
    SUM(IF(age < 20,1,0)) as 'Under 20',
    SUM(IF(age BETWEEN 20 and 29,1,0)) as '20 - 29',
    SUM(IF(age BETWEEN 30 and 39,1,0)) as '30 - 39',
    SUM(IF(age BETWEEN 40 and 49,1,0)) as '40 - 49',
    SUM(IF(age BETWEEN 50 and 59,1,0)) as '50 - 59',
    SUM(IF(age BETWEEN 60 and 69,1,0)) as '60 - 69',
    SUM(IF(age BETWEEN 70 and 79,1,0)) as '70 - 79',
    SUM(IF(age >=80, 1, 0)) as 'Over 80',
    SUM(IF(age IS NULL, 1, 0)) as 'Not Filled In (NULL)'

FROM (SELECT TIMESTAMPDIFF(YEAR, birth_date, CURDATE()) AS age FROM inquiries) as derived