对于每位乘客，获取同一购物车中有多少其他乘客的信息

Question

I have a table;

INSERT INTO passengers(id, name, cart_id)
VALUES (1, Uolevi, 1), (2,Maija, 1), (3, Kaaleppi, 2), (4, Kotivalo, 4), (5, Juustina, 4), (6, Vihtori, 4) 

Expected result:

| Column A | Column B |
| -------- | -------- |
| Uolevi   |   1      |
| Maija    |   1      |
| Kaaleppi |   0      |
| Kotivalo |   2      |
| Juustina |   2      |
| Vihtori  |   2      |

I used COUNT(*) and GROUP BY cart_id. Then the column Name only shows the first name of each cart_id.

Answer 1

See http://sqlfiddle.com/#!18/d4b80e/6/0

select
  p1.id
  ,p1.name
  ,(select count(*) from Passengers p2 
      where p1.cart_id = p2.cart_id and p1.Id <> p2.Id ) as CountOfOthers
from
  Passengers p1

Basically the sub query gets the count of passengers with the same [Passenger.Cart_Id] excluding themselves.

Output

id  name        CountOfOthers
1   Uolevi      1
2   Maija       1
3   Kaaleppi    0
4   Kotivalo    2
5   Juustina    2
6   Vihtori     2