st: Merge/append datasets (?)

["Roy,Suryadipta" <suryadipta.roy@lawrence.edu>]

Hi,

I have two datasets (a.dta and b.dta) as follows:

a.dta
id	oid	did	x3
1	1	1	5
1	1	2	6
1	1	3	6
1	1	4	6
2	2	1	6
2	2	2	6
2	2	3	6
2	2	4	6
3	3	1	6
3	3	2	6
3	3	3	6
3	3	4	6
4	4	1	6
4	4	2	6
4	4	3	6
4	4	4	6

b.dta has observation missing for id=1:
id	x1
1	.
2	12
3	13
4	14

I am trying to merge the two datasets such that the new 
dataset should read as:
id	oid	did	x3	x1
1	1	1	5	.
1	1	2	6	.
1	1	3	6	.
1	1	4	6	.
2	2	1	6	12
2	2	2	6	12
2	2	3	6	12
2	2	4	6	12
3	3	1	6	13
3	3	2	6	13
3	3	3	6	13
3	3	4	6	13
4	4	1	6	14
4	4	2	6	14
4	4	3	6	14
4	4	4	6	14

The common variable in both is "id" and both have already 
been sorted by "id". I am getting the error message as 
"using data not sorted" or "variable id does not uniquely 
identify observations in the master data".
Any help would be highly appreciated!!!
Thanks,
Suryadipta.
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