I've spent ages researching this and found nothing that directly addresses the question.

I’m trying to see which customers are ‘like’ other customers at an aggregate level.

How do I count the number of duplicate values on one column for certain values of another column?

In the example below, I’d like to know how many records for ‘Customer 112’ and ‘Customer 113’ have duplicate variable values to those shown for “Customer 111”.

The answer is: Customer 112 = 3 (27, 28 and 30 are all duplicates of values shown for Customer 111) and Customer 113 = 2 (24 and 26 are both duplicates of values shown for Customer 111).

Customer Variable
111      21
111      22
111      23
111      24
111      26
111      27
111      28
111      29
111      30
112      23
112      27
112      28
112      30
112      31
112      33
112      35
113      24
113      26
113      33
113      35

The output would be:

Customer  Count
112        3
113        2

Any suggestions would be greatly appreciated.

Here is one way to do it, by joining to the "111" customer values and then aggregating:

select t.customer, count(t111.variable) as "count"
from t left outer join
     (select t.*
      from t
      where customer = 111
     ) t111
     on t.variable = t111.variable
group by t.customer;

I think the above is clear on what it is doing. However, you can eliminate the subquery (which is nice in MySQL):

select t.customer, count(t111.variable) as "count"
from t left outer join
     t t111
     on t.variable = t111.variable and t111.customer = 111
group by t.customer;