I have data that looks like this:

A B
1 Account Notes
--------- ------
2 1000 abc
3 def
4 ghi
5 1001 jkl
6 mno
7 1000 pq
8 rs
9 ... ...

Is there any way to add in a third column that concatenates the notes rows
into one row? I know doing this: =B2 & "
" &B3& "
" &B4 in cell C2
will give me what I want, but I have about 12,000 rows to sort through and I
don't want to do this all manually. Is there a way to create intermediate
columns with dummy variables to say, if A is null, but A+1 is not null,
concatenate column B for all the rows for a particular account?

Thanks.