SQL - 来自 3 个不同表的联合计数（*）导致笛卡尔积

Question

我有 3 个表格：帖子、图片和相册。 我需要计算每年发表的帖子、图片和相册的数量。 这里是表的创建：

create table Posts
(
 PostID int primary key identity,
 Description varchar(30) not null,
 Date smalldatetime not null,
 UserWriterID int references Users(UserID)
)

create table Albums
(
 AlbumID int primary key identity,
 Name varchar(20) not null,
 Description varchar(30) not null,
 Date smalldatetime not null,
 AlbumOwnerID int references Users(UserID) 
)

create table Pictures
(
 PictureID int primary key identity,
 AlbumID int references Albums(AlbumID),
 Description varchar(30) not null,
 Date smalldatetime not null,
)

表格中的一些数据样本：

图片：

PictureID | AlbumID | Desc | Date

    1    |   1   |   aa   | 2000-01-01 00:00:00

    2    |   1   |   bb   | 2011-08-31 15:24:00

    10   |   2   |   d    | 2010-07-17 12:35:00

专辑：

AlbumID | Name |    Desc    |    Date     |      AlbumOwnerID

1   |   My album | my new album |  2000-01-01 00:00:00 |    2

2   |   Belgium trip    | trip in Belgium |  2005-07-08 00:00:00 |  2

9   |    Work   | Work and I  | 2011-07-08 00:00:00  |  11

帖子：

PostID  | Desc | Date | UserWriterID

1  |  Feeling good |    2013-09-10 07:44:00 |  2

2   |  FUN FUN FUN |  2015-12-21 09:45:00  |    8

3   |   Whats up?   | 2014-01-18 12:54:00 |     7 

然后我插入数据。

现在，我编写了 3 个查询来获取每个表每年的计数。

create view PostPerYear as
select YEAR(Date) as Year , count(PostID) as Posts
from Posts
group by YEAR(Date)
go 

create view AlbumsPerYear as
select YEAR(Date) as Year , count(AlbumID) as Albums
from Albums 
group by YEAR(Date)
go

create view PicturesPerYear as
select YEAR(Date) as Year , count(PictureID) as Pictures
from Pictures
group by YEAR(Date)
go

但是，我需要一个能显示以上所有结果的查询TOGETHER。 例如：

年份 |帖子数 |专辑数量 |图片计数

2013 | 3 | 4 | 1

2005 | 13 |23 | 5

当我尝试时，我得到了笛卡尔积，例如：

select (p.Year) , (p.Posts) ,  (a.Albums) , (pic.Pictures)
from PostPerYear p ,AlbumsPerYear a ,PicturesPerYear pic
group by p.Year, p.Posts , a.Albums , pic.Pictures

结果是： 年份|帖子 |相册 |图片

2013    9   1   1

2013    9   1   2

2013    9   1   3

2013    9   1   4

2013    9   1   5

2013    9   1   6

2013    9   1   11

2013    9   2   1

2013    9   2   2

2013    9   2   6

2013    9   2   11

2014    10  1   1

2014    10  1   2

2014    10  1   3

2014    10  1   4

2014    10  1   5

2014    10  1   6

And Continued....
------------------------------
------------------------------

请问答案是什么？

谢谢！

Answer 1

在FROM 子句中指定多个表相当于执行CROSS JOIN。你可以使用UNION ALL来解决这个问题。

WITH summary(year, posts, albums, pictures) AS (        
    SELECT year, posts, 0, 0 FROM PostsPerYear        
    UNION ALL        
    SELECT year, 0, albums, 0 FROM AlbumsPerYear        
    UNION ALL        
    SELECT year, 0, 0, pictures FROM PicturesPerYear        
)        
SELECT year,        
       sum(posts)    AS posts,        
       sum(albums)   AS albums,        
       sum(pictures) AS pictures        
FROM summary        
GROUP BY year;

Answer 2

首先是简化的 DDL 和一些示例数据：

create table Posts
(
 PostID int primary key identity,
 Date smalldatetime not null
)

create table Albums
(
 AlbumID int primary key identity,
 Date smalldatetime not null
)

create table Pictures
(
 PictureID int primary key identity,
 Date smalldatetime not null,
)




create view PostPerYear as
select YEAR(Date) as Year , count(PostID) as Posts
from Posts
group by YEAR(Date)
go 

create view AlbumsPerYear as
select YEAR(Date) as Year , count(AlbumID) as Albums
from Albums 
group by YEAR(Date)
go

create view PicturesPerYear as
select YEAR(Date) as Year , count(PictureID) as Pictures
from Pictures
group by YEAR(Date)
go


insert posts values('20130101'), ('20140202')
insert albums values('20130101'), ('20140202'), ('20140202')
insert pictures values('20130101'), ('20130101'), ('20140202')
go

现在，使用外连接（我建议使用 ANSI 连接语法）：

select coalesce(p.Year, a.year, pic.year) as year, p.Posts, a.Albums, pic.Pictures
from PostPerYear p 
full outer join AlbumsPerYear a on p.year = a.year
full outer join PicturesPerYear pic on p.year = pic.year

返回这个：

Year    Posts   Albums  Pictures
2013    1   1   2
2014    1   2   1