sql - How to calculate time difference between current and previous row in MySQL -

i have mysql table t1 :

what want do calculations between rows , save value in new coloumn called diff

ticketid| datenew  | diff 16743     12:36:46   0 16744     12:51:25   15.  minute   16745     12:57:25   6.5  minute .......... ....... etc 

i know there similar questions ,but ive tried of solutions posted here no success,so how solve query ???

to time difference in minutes between current , previous row, can use timestampdiff on datenow , previous time, can via subquery:

select ticketid, datenew,     timestampdiff(minute,datenew,(select datenew mytable t2         t2.ticketid < t1.ticketid order t2.ticketid desc limit 1)) diff mytable t1 

update

here's way using variable store previous datenew value might faster:

select ticketid, datenew, timestampdiff(minute,datenew,prevdatenew) (     select ticketid, datenew, @prevdatenew prevdatenew,        @prevdatenew := datenew     mytable order ticketid ) t1