How do I do a count formula

[#1]

I have a data set that had 19 million rows.  Within that data set I want to count the number of instruments a customer has.  I have the customer data in one column and the instruments in another column.  My goal is to select customers that only have 1 instrument.  

Thanks 

[#2]

I suggest that you use 

     Tables==>Summary

grouping on (Customer, Instruments)

and then on the summary table repeat

     Tables==>Summary

grouping on (Customer)

the table it produces will have the number of instruments,

You can then easily run

     Tables==>Update

to join the counts back into the original table

[#3]

Thanks so much,  This helps.