题解 | #计算25岁以上和以下的用户数量#

解法一：IF语句

SELECT if(age<25 OR age IS null,"25岁以下","25岁及以上") as age_cut,count(u.device_id) as number
FROM user_profile as u
GROUP BY age_cut

解法二：UNION + ALL并集显示

SELECT "25岁以下" as age_cut,count(device_id)
FROM user_profile
WHERE age<25 OR age IS null
UNION ALL
SELECT "25岁及以上" as age_cut,count(device_id)
FROM user_profile
WHERE age>=25

解法三：CASE语句

SELECT 
    CASE
        WHEN age<25 OR age IS null THEN "25岁以下"
        else "25岁及以上"
    END AS age_cut,
    count(device_id) AS number
FROM user_profile
GROUP BY age_cut