How to generate group mean variables for each time?

[Sue Lee]

My data is as now

code	pool	time	price
a1	j1	20190101	1
a1	j1	20190102	2
a1	j2	20180101	1
a1	j2	20190101	1
a2	j1	20190101	2
a2	j1	20190102	3
a2	j2	20180101	1

What I want to get is the average of the price of each pool for each time value.

pool	time	price_avg
j1	20190101
j1	20190102
j2	20180101
j2	20190101

By command "bysort pool : egen price_avg = mean(price)" I only got the one average value of price over times (such as 2 for every values in pool j1.)
Instead of this, what command should I have to use?

[Nick Cox]

bysort pool time :

[Sue Lee]

Thank you so much!