計數列的值等於行ID

Question

我有這樣的表。

id | name | desc | parent 
------------------------- 
1 | abc | def | 0 
2 | abc | def | 1 
3 | abc | def | 1 
4 | abc | def | 1 
5 | abc | def | 2 
6 | abc | def | 3 

，我需要得到的是這一點，讓所有行數多少行parent同實際行的ID。而且所有的都必須按這個數量排序。

id | name | desc | parent | count 
------------------------- 
1 | abc | def | 0  | 3 
2 | abc | def | 1  | 1 
3 | abc | def | 1  | 1 
4 | abc | def | 1  | 0 
5 | abc | def | 2  | 0 
6 | abc | def | 3  | 0 

我這個結束了，但實際上它沒有工作：/

select c.pocet, a.* from posts a, (select count(*) as pocet from posts b where b.parent=a.id) c 

感謝您的幫助！

Answer 1

試試這個例子。根據需要添加更多列。

select 
t.id,t.parent,coalesce(x.cnt,0) as cnt 
from t left join 
(select parent,count(*) as cnt 
from t 
group by parent) x 
on x.parent = t.id 

Fiddle

Answer 2

你實際上是接近，你只需要切換到標量子查詢：

select a.*, 
    (select count(*) from posts b where b.parent=a.id) as pocet 
from posts a