creating mean variable by groups

[leonh]

I apologize in advance if this question is silly, but how do I create a mean variable by groups. Take the below table for example, I want to calculate a Var2 such that for the first 3 observations, Var2 is the mean(Var1 of the first 3 obs), and Var2 for the next 7 obs is mean(Var1 from id=4 to id=10). ie, to get from Table 1 to Table 2. Thank you!

Table 1
ID	Group	Var1
1	A	1
2	A	1
3	A	4
4	B	3
5	B	3
6	B	3
7	B	3
8	B	3
9	B	3
10	B	10

Table 2
ID	Group	Var1	Var2
1	A	1	2
2	A	1	2
3	A	4	2
4	B	3	4
5	B	3	4
6	B	3	4
7	B	3	4
8	B	3	4
9	B	3	4
10	B	10	4

[LinusH · Accepted solution]

SQL with a GROUP BY that is remerging the statistic with the original query:

select group, var1, mean(var1) as var2

from table1

group by group;