POSTGRESQL中的每個用戶獲得每個用戶的計數

Question

我是新來的postgresql。我有一個數據庫中有三個表：用戶，訂單，評論。這三個表看起來像這樣

 Orders    Comments 
     ------    -------- 
     ID     ID 
     UserID    UserID 
     Description   Details 
     CreatedOn    CreatedOn 

我試圖讓我的所有用戶，有多少訂單每個用戶取得了多少評論的每用戶作出的列表。換句話說，查詢的結果應該是這樣的：

UserID  Orders Comments 
------  ------ -------- 
1   5  7 
2   2  9 
3   0  0 
... 

目前，我嘗試以下：

SELECT 
    UserID, 
    (SELECT COUNT(ID) FROM Orders WHERE UserID=ID) AS Orders, 
    (SELECT COUNT(ID) FROM Comments WHERE UserID=ID) AS Comments 
FROM 
    Orders o, 
    Comments c 
WHERE 
    o.UserID = c.UserID 

這是做這種類型的查詢的正確方法嗎？還是有人可以從性能角度提供更好的方法？

Answer 1

SQL Fiddle

select 
    id, name, 
    coalesce(orders, 0) as orders, 
    coalesce(comments, 0) as comments 
from 
    users u 
    left join 
    (
     select userid as id, count(*) as orders 
     from orders 
     group by userid 
    ) o using (id) 
    left join 
    (
     select userid as id, count(*) as comments 
     from comments 
     group by userid 
    ) c using (id) 
order by name 

Answer 2

通常的方法做，這是通過使用外部由id（和名稱）連接到其他兩個表，然後組

select u.id, 
     u.name, 
     count(distinct o.id) as num_orders, 
     count(distinct c.id) as num_comments 
from users u 
    left join orders o on o.userId = u.id 
    left join comments c on c.userId = u.id 
group by u.id, u.name 
order by u.name; 

這很可能是比你的方法快。但Postgres的查詢優化器非常聰明，我已經看到兩種解決方案在性能上基本相同的情況。

您需要對數據進行測試，並查看執行計劃以確定哪一個更有效。