MySQL calculates the average user order cycle

This Thursday, a colleague asked me the table structure as follows:

Calculate the average user's next single cycle, how to calculate, start looking at, I also have no train of thought, how simple calculation. This morning was also carried out under a try,

Here is my calculation of SQL, for reference only, if there is an error or a better way, welcome to communicate together,

Select  C.id,avg (GAP) from (          select A.id,a.date as D1,min (b.date) as D2,timestampdiff (Day,a.date,min (b.date)) As Gap from User_order as  a left joins  User_order as B on a.id = b.ID and A.date < b.date where b.date are not nu ll GROUP by A.id,a.date) as C Group by C.id;

　　

MySQL calculates the average user order cycle