class InMemoryDatabase:
    def _init_(self):
        self.database = {}

    def set(self, timestamp, key, field, value):
        if key not in self.database:
            self.database[key] = {}
        self.database[key][field] = value

    def compare_and_set(self, timestamp, key, field, expected_value, new_value):
        if key in self.database and field in self.database[key]:
            current_value = self.database[key][field]
            if current_value == expected_value:
                self.database[key][field] = new_value
                return "true"
        return "false"

    def compare_and_delete(self, timestamp, key, field, expected_value):
        if key in self.database and field in self.database[key]:
            current_value = self.database[key][field]
            if current_value == expected_value:
                del self.database[key][field]
                return "true"
        return "false"

    def get(self, timestamp, key, field):
        if key in self.database and field in self.database[key]:
            return self.database[key][field]
        return None

    def scan(self, timestamp, key):
        if key in self.database:
            fields = sorted(self.database[key].keys())
            return ", ".join([f"{field}({self.database[key][field]})" for field in fields])
        return ""

    def scan_by_prefix(self, timestamp, key, prefix):
        if key in self.database:
            fields = sorted(filter(lambda f: f.startswith(prefix), self.database[key].keys()))
            return ", ".join([f"{field}({self.database[key][field]})" for field in fields])
        return ""

# Example usage:
db = InMemoryDatabase()
queries = [
    ["SET", "0", "A", "BC", "4"],
    ["SET", "10", "A", "BD", "5"],
    ["SET", "20", "A", "C", "6"],
    ["SCAN_BY_PREFIX", "30", "A", "B"],
    ["SCAN", "40", "A"],
    ["SCAN_BY_PREFIX", "50", "B", "B"]
]

for query in queries:
    operation = query[0]
    if operation == "SET":
        db.set(*query[1:])
    elif operation == "COMPARE_AND_SET":
        print(db.compare_and_set(*query[1:]))
    elif operation == "COMPARE_AND_DELETE":
        print(db.compare_and_delete(*query[1:]))
    elif operation == "GET":
        print(db.get(*query[1:]))
    elif operation == "SCAN":
        print(db.scan(*query[1:]))
    elif operation == "SCAN_BY_PREFIX":
        print(db.scan_by_prefix(*query[1:]))