日期相同的数据a求平均，b求平均

 格式化一下日期，每个数据库都有自带的函数，格式化到天。
 然后在select avg(a),avg(b) from 表 group by 格式化后的日期

SELECT avg(a) ,avg(b) from (select * from table t where t.updatetime BETWEEN '2016-04-12 00:00:00' and '2016-04-12 23:59:59') a;

SELECT AVG(a),AVG(b) FROM table GROUP BY CONVERT(char(10), updaterime, 111)；

按照日期进行分组然后进行平均值运算即可

 select 
    date_format(t.updatetime,'%Y-%m-%d') as 'time',
    count(high_pressure)

from t_web_data_tonometer t
GROUP BY time

      select 
    date_format(t.updatetime,'%Y-%m-%d') as 'time',
    AVG(a),
    AVG(b)

from t_web_data_tonometer t
GROUP BY time