st: Counting unique strings

["Paul O'Brien" <paul.obrien@afe2.org.uk>]

Hi,

I have a database of 40,000 clinic visits, with multiple visits by each 
patient (patient_id). How do I count the number of patients seen?

Thanks

patient_id
CGHC:FP:0002
CGHC:FP:0002
CGHC:FP:0002
CGHC:FP:0002
CGHC:FP:00144
CGHC:FP:00144
CGHC:FP:00150
CGHC:FP:00150
CGHC:FP:00150
CGHC:FP:00150
CGHC:FP:00155
CGHC:FP:00155
CGHC:FP:00159
CGHC:FP:00159
CGHC:FP:00160
CGHC:FP:00160
CGHC:FP:00162
CGHC:FP:00162
CGHC:FP:00163
CGHC:FP:00163
CGHC:FP:00175
CGHC:FP:00175
CGHC:FP:0018
CGHC:FP:0018
CGHC:FP:0018
CGHC:FP:00185
CGHC:FP:00185
CGHC:FP:0020
CGHC:FP:0020
CGHC:FP:00217
CGHC:FP:00219
CGHC:FP:00219

Dr Paul O'Brien
Westside Contraceptive Services
Raymede Clinic, Exmoor St, London
W10 6DZ
020 8962 4450 - tel
020 8962 4451 - fax
07900 495 118 - mob

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