DELETE MIN??

USE tempdb;

CREATE TABLE dbo.Patients(
NUMPAT int NOT NULL,
TREATMENT varchar(15) NOT NULL,
DATE datetime NOT NULL
);

INSERT dbo.Patients VALUES(1, 'Treatment 1', '20080101');
INSERT dbo.Patients VALUES(1, 'Treatment 2', '20080102');
INSERT dbo.Patients VALUES(1, 'Treatment 3', '20080103');
INSERT dbo.Patients VALUES(2, 'Treatment 4', '20080201');
INSERT dbo.Patients VALUES(2, 'Treatment 5', '20080202');
INSERT dbo.Patients VALUES(3, 'Treatment 6', '20080301');
INSERT dbo.Patients VALUES(3, 'Treatment 7', '20080302');
INSERT dbo.Patients VALUES(3, 'Treatment 8', '20080303');
INSERT dbo.Patients VALUES(3, 'Treatment 9', '20080304');

DELETE dbo.Patients 
FROM dbo.Patients T
WHERE T.DATE = (
    SELECT MIN(Q.DATE) 
    FROM dbo.Patients Q 
    WHERE T.NUMPAT = Q.NUMPAT
);

/* Output:

(3 row(s) affected)

*/

SELECT *
FROM dbo.Patients;

/* Output:

NUMPAT      TREATMENT       DATE
----------- --------------- -----------------------
1           Treatment 2     2008-01-02 00:00:00.000
1           Treatment 3     2008-01-03 00:00:00.000
2           Treatment 5     2008-02-02 00:00:00.000
3           Treatment 7     2008-03-02 00:00:00.000
3           Treatment 8     2008-03-03 00:00:00.000
3           Treatment 9     2008-03-04 00:00:00.000

(6 row(s) affected)

*/

DROP TABLE dbo.Patients;