CREATE TABLE "DBA"."Table1" (
	"Id" integer NOT NULL ,
	"NPerson" tinyint NOT NULL,
	"Date_Open" date NULL,
	"Date_close" date NULL,
PRIMARY KEY CLUSTERED ( "Id",  "NPerson" )

insert into table1 (id,nperson,date_open,date_close)
select  1 , 1 ,null,null
union 
select  1 , 2 ,'20040101',null
union 
select  1 , 3 ,'20040110','20040130'
union
select  1 , 4 ,'20040120',null

id   count(id)   date1         date2
 1      2           '20040101'   '20040109'
 1      3           '20040110'   '20040119'
 1      4           '20040120'   '20040130'
 1      3           '20040131'   '20040131'