BEGIN;
  TYPE Date UNION;
  TYPE DateValid IS {Date POSSREP {year INTEGER, month INTEGER, day INTEGER}};
  TYPE DateNone IS {Date POSSREP {}};
  TYPE DateUnknown IS {Date POSSREP {}};
END;

VAR patient REAL RELATION {id INT, lastname CHAR} KEY {id};

INSERT patient RELATION
{TUPLE {id 1001, lastname 'Hopper'},
 TUPLE {id 4004, lastname 'Wirth'},
 TUPLE {id 3003, lastname 'Kemeny'},
 TUPLE {id 2002, lastname 'Gosling'},
 TUPLE {id 5005, lastname 'Kurtz'}
 };

VAR visit REAL RELATION {id INT, date Date, score RATIONAL} KEY {id, date};

INSERT visit RELATION
{
TUPLE {id 2002, date DateValid(2020,09,10), score 6.8},
TUPLE {id 1001, date DateValid(2020,09,17), score 5.5},
TUPLE {id 4004, date DateValid(2020,09,24), score 8.4},
TUPLE {id 2002, date DateValid(2020,10,08), score NAN},
TUPLE {id 1001, date DateNone(), score 6.6},
TUPLE {id 3003, date DateValid(2020,11,12), score NAN},
TUPLE {id 4004, date DateValid(2020,11,05), score 7.0},
TUPLE {id 1001, date DateValid(2020,11,19), score 5.3}
};

((SUMMARIZE (visit WHERE score>0.0) BY {id}: {sumscore := SUM(score), avgscore := AVG(score)}) UNION
 (EXTEND (patient {id} MINUS ((visit WHERE score>0.0) {id})): {sumscore := NaN, avgscore := NaN})) JOIN
    (SUMMARIZE visit BY {id}: {maxdate := MAX(date)} UNION
     (EXTEND (patient {id} MINUS (visit {id})): {maxdate := DateUnknown()})) JOIN
    patient