# if判断
SELECT IF(age<25 OR age IS NULL,'25岁以下','25岁以及上') age_cut,COUNT(device_id) Number
FROM user_profile
GROUP BY age_cut

# 联合查询
SELECT '25岁以下' age_cut,COUNT(device_id) Number
FROM user_profile
WHERE age < 25 OR age IS NULL
UNION
SELECT '25岁以及上' age_cut,COUNT(device_id) Number
FROM user_profile
WHERE age >= 25