MySQL is updated as the result of the result set subtraction

1. Example 

Calculate the following distance under the same timestamp 

4 car following distance = 94.84-72.48 

2.demo

update cacc_log e,(select timestamp,x from 
(
	select timestamp,sum(x) as x from
		(
		select timestamp,x from cacc_log where carid ='5' and rwmc ='CACC01'
		union all
		select timestamp,x*(-1) from cacc_log where carid ='4' and rwmc ='CACC01'
		) 
	as c group by timestamp
) as d ) f
set e.flowdist = f.x 
where e.timestamp = f.timestamp and carid ='4' and rwmc ='CACC01';