How to SUM by parent table value while ignoring child table cardinality?

I am looking for a way to extract data with a single query and answering to two fundamental questions:

• I want to be able to get details about a child table where I have to group by a column and SUM amounts, and
• I want to keep a "global" sum from the parent table valid.

Suppose the following tables:

CREATE TABLE Invoice (
  Id        VARCHAR(3) PRIMARY KEY,
  Pax       INT NOT NULL
)

CREATE TABLE InvoiceLine (
  Id        VARCHAR(3) PRIMARY KEY,
  InvoiceId VARCHAR(3) NOT NULL,
  Activity  VARCHAR(80) NOT NULL,
  Amount    DECIMAL(19,4) NOT NULL,
  CONSTRAINT FK_InvoiceLine_InvoiceId FOREIGN KEY (InvoiceId) REFERENCES Invoice (Id)
)

And the following datas:

Invoice Table:

InvoiceLine Table:

    INSERT INTO Invoice VALUES ('001', 2);
    INSERT INTO Invoice VALUES ('002', 2);
    INSERT INTO Invoice VALUES ('003', 4);
    INSERT INTO InvoiceLine VALUES ('001', '001', 'Ticketing',   40);
    INSERT INTO InvoiceLine VALUES ('002', '001', 'Reservation', 10);
    INSERT INTO InvoiceLine VALUES ('003', '002', 'Ticketing',   30);
    INSERT INTO InvoiceLine VALUES ('004', '002', 'Ticketing',   30);
    INSERT INTO InvoiceLine VALUES ('005', '002', 'Reservation',  8);
    INSERT INTO InvoiceLine VALUES ('006', '002', 'Insurance',    6.6);
    INSERT INTO InvoiceLine VALUES ('007', '003', 'Ticketing',  120);
    INSERT INTO InvoiceLine VALUES ('008', '003', 'Ticketing',   40);

My goal is to have a query to retrieve (in the same query):

1. Amount: sum of amount per activity
2. Pax: number of pax per activity
3. GlobalPax: number of pax served globally (sum = 8)

I have found a way to obtain the correct result, but it requires to group by i.Id, which increase the number of rows returned (in my case I have millions of rows).

SELECT
  i.Id,
  il.Activity,
  SUM(il.Amount) AS Amount,
  AVG(i.Pax) AS Pax,
  AVG(CAST(i.Pax AS float)) / COUNT(*) OVER (PARTITION BY i.Id) AS GlobalPax
FROM
  Invoice AS i
  JOIN InvoiceLine AS il ON i.Id = il.InvoiceId
GROUP BY
  i.Id,
  il.Activity

Which gives me:

The results is almost perfect, but I don't need the Id column (required for the PARTITION BY clause).

A perfect result could be:

Thanks

You should apply next grouping by Activity, if don't need the Id column.

See example

select Activity
  ,sum(Amount) as Amount
  ,sum(Pax) as Pax
  ,sum(GlobalPax) as GlobalPax
from( -- your query
SELECT
  i.Id,
  il.Activity,
  SUM(il.Amount) AS Amount,
  AVG(i.Pax) AS Pax,
  AVG(CAST(i.Pax AS float)) / COUNT(*) OVER (PARTITION BY i.Id) AS GlobalPax
FROM
  Invoice AS i
  JOIN InvoiceLine AS il ON i.Id = il.InvoiceId
GROUP BY i.Id, il.Activity
)a
group by Activity

fiddle