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Returning Count from database table joined with another table

开发者 https://www.devze.com 2023-03-04 06:41 出处:网络
I have two tables Titles ID Name 1\'Title 1\' 2\'Title 2\' 3\'Title 3\' Comments ID TitleID Comment 11开发者_StackOverflow中文版\'Title 1 Comment #1\'

I have two tables

Titles

ID Name
1  'Title 1'
2  'Title 2'
3  'Title 3'

Comments

ID TitleID Comment
1   1       开发者_StackOverflow中文版'Title 1 Comment #1'
2   1       'Title 1 Commnet #2'
3   2       'Title 2 comment'

I want a recordset that returns all the titles in Titles and the number of comments for that title, as such

 Name    # Comments
 Title 1     2
 Title 2     1
 Title 3     0

I worked out how to return all the titles and each comment, but I am having difficulty returning the data with just the count of comments. (My query is below)

SELECT T.ID, T.Name, C.Comment
FROM Titles T
LEFT OUTER JOIN Comments C ON T.ID = C.TitleID
  UNION
SELECT T.TitleID, T.Name, C.Comment
FROM Titles T
RIGHT OUTER JOIN Comments C ON T.ID = C.TitleID

Any help would be greatly appreciated.

update

I forgot that I also wanted to return titles without comments. I updated the examples above.


SELECT t.Name as Title, COUNT(c.ID) as NumberOfComments
FROM Titles t
LEFT JOIN Comments c ON c.TitleID = t.ID
GROUP BY t.Name

Ideally you would want to group by the TitleID but you would have to include that in your selection as well. If you're able to ignore that column (or use it) then I would use that instead of t.Name (or alongside t.Name).


Can also be done with a subselect.

Select T.Name,
       (Select Count(*)From Comments C Where C.TitleID=T.ID)As NumComments
  From Titles T

The advantage is that no GROUP BY is needed. This can be important if tables in your real data have a lot more columns than the example above.


SELECT title.name, count(comments.comment) as NumComments
FROM titles,comments
WHERE title.id = comment.titleID
Group By title.name
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