Merge question

[joe4444]

I have a simple merging question that should be an easy answer, but I can not figure it out. Here is the data. Thanks.

Dataset1

ID Var1

1   Yes

2

2

3   No

Dataset2

ID Var1

1  

2  Yes

3  

data dataset3;

merge dataset1 (in=a) dataset2 (in=b);

by ID;

if a;

run;

Dataset3

ID Var1

1   Yes

2   Yes

2

3   No

Why does ID = 2 not have "Yes" replace the blanks in Var1 for both records?

Thanks.

[Tom]

Because only one record with a YES was provided from the second dataset.  So on the second observation for ID=2 only dataset1 is contributing a value.

Many to One merges like this only work when the non-key variables do not overlap.