10.条件函数：case when
--如：select case when age<20 then "20岁以下"
when age>=20 and age<30 then "20~30岁"
when age>=30 and age<40 then "30~40岁"
else "40岁以上" end as age_type,
count (distinct user_id) user_num
from user_info
group by ...;
11.if函数
if(条件表达式，结果1，结果2) ：当条件为真-->结果1，否则结果2
--如：select if (level>5,"高","低") [as level_type] from...
12.对json字符串和map类型的处理
get_json_object(string json_string,string path)
string json_string:列名
string path：用$.key取值
--如：字段： extra1(string): {"systemtype":"ios","education":"master","marriage_status":"1","phone brand":"iphone X"}
--字段： extra2(map<string,string>): {"systemtype":"ios","education":"master","marriage_status":"1","phone brand":"iphone X"}
对于json类型：
例如：
SELECT get_json_object(extra1, ‘$.phonebrand‘) as phone_brand,
count(distinct user_id) user_num
FROM user_info
GROUP BY get_json_object(extra1, ‘$.phonebrand‘);
对于map类型：
例如：select extra2[‘phonebrand‘] as phone_brand,
count(distinct user_id) user_num
FROM user_info
GROUP BY extra2[‘phonebrand‘];
