Question

My database isn’t actually customers and orders, it’s customers and prescriptions for their eye tests (just in case anyone was wondering why I’d want my customers to make orders less frequently!)

I have a database for a chain of opticians, the prescriptions table has the branch ID number, the patient ID number, and the date they had their eyes tested. Over time, patients will have more than one eye test listed in the database. How can I get a list of patients who have had a prescription entered on the system more than once in six months. In other words, where the date of one prescription is, for example, within three months of the date of the previous prescription for the same patient.

Sample data:

Branch  Patient DateOfTest 1      1          2007-08-12 1      1          2008-08-30 1      1          2008-08-31 1      2          2006-04-15 1      2          2007-04-12 

I don’t need to know the actual dates in the result set, and it doesn’t have to be exactly three months, just a list of patients who have a prescription too close to the previous prescription. In the sample data given, I want the query to return:

Branch   Patient 1       1 

This sort of query isn’t going to be run very regularly, so I’m not overly bothered about efficiency. On our live database I have a quarter of a million records in the prescriptions table.

Answer 1

Something like this

select p1.branch, p1.patient from prescription p1, prescription p2 where p1.patient=p2.patient and p1.dateoftest > p2.dateoftest and datediff('day', p2.dateoftest, p1.dateoftest) < 90; 

should do… you might want to add

and p1.dateoftest > getdate() 

to limit to future test prescriptions.