Question

我有两张桌子，如下：

#Articles:
ID | Title
1    "Article title"
2    "2nd article title"

#Comments:
ID | ParentID | Comment
1    1          "This is my comment"
2    1          "This is my other comment"

我一直想知道，获得以下结果的最优雅方式是什么：

ID | Title |          NumComments
1    "Article title"      2
2    "2nd article title"  0

这适用于SQL Server。

Answer 1

这通常比子查询方法更快，但一如既往，您必须对系统进行分析以确保：

SELECT a.ID, a.Title, COUNT(c.ID) AS NumComments
FROM Articles a
LEFT JOIN Comments c ON c.ParentID = a.ID
GROUP BY a.ID, a.Title

Answer 2

select title, NumComments = (select count(*) 
from comments where parentID = id) from Articles

Answer 3

SELECT 
   A.ID, A.Title, COUNT(C.ID) 
FROM 
   Articles AS A 
LEFT JOIN 
   Comments AS C ON C.ParentID = A.ID 
GROUP BY 
   A.ID, A.Title 
ORDER BY 
   A.ID

Answer 4

我会这样做：

select a.ID 'ArticleId',
       a.Title,
       count(c.ID) 'NumComments'
from   Articles a
left join
       Comments c
on     a.ID = c.ParentID
group by a.ID, a.Title

这可能有助于决定加入或使用子查询：

Transact-SQL - sub query or left-join?

Answer 5

SELECT
Articles.ID
,Articles.TItle
,(SELECT Count(*) FROM Comments WHERE Comments.ParentId = Artices.ID) AS CommentCount
FROM Articles

Answer 6

SELECT Articles.Title，COUNT（Comments.ID） FROM Articles INNER JOIN关于Articles.ID = Comments.ParentID的评论 GROUP BY Articles.Title

SQL Query获取一行，以及关联行的计数

6 个答案: