Sort out various SQL statements that are commonly used in mysql

Sort out various SQL statements that are commonly used in mysql

① Statistics are made by day and month, and daily trend changes are required.
SQL queries the sum of values of all previous values on a daily (monthly) basis.
Implementation: Compare op_time with a. op_time.

Select DAY (op_time), (select sum (price) from t_margin_profit where day (op_time) <= day (. op_time) and year (op_time) = '20140901' and month (op_time) = '1') from t_margin_profit a where year (op_time) = '20140901' and month (op_time) = '1' group by day (op_time)

② To be continued...