使用表自己的数据更新包含组的表

Question

希望我能正确解释。这是一些示例代码。我想要一些行数，其中几列中的值匹配。

DECLARE @myTemp TABLE
(
        Author varchar(20),
        Title varchar(25),
        MediaType varchar(5),
        ItemCount int NULL
)

INSERT INTO @myTemp (Author, Title, MediaType) VALUES ('Heinlein','The Roads Must Roll', 'CD')
INSERT INTO @myTemp (Author, Title, MediaType) VALUES ('Heinlein','The Roads Must Roll', 'Book')
INSERT INTO @myTemp (Author, Title, MediaType) VALUES ('Heinlein','Time Enough for Love', 'Book')
INSERT INTO @myTemp (Author, Title, MediaType) VALUES ('Asimov','The Foundation Trilogy', 'CD')
INSERT INTO @myTemp (Author, Title, MediaType) VALUES ('Asimov','The Foundation Trilogy', 'Book')
INSERT INTO @myTemp (Author, Title, MediaType) VALUES ('Asimov','I Robot', 'CD')
INSERT INTO @myTemp (Author, Title, MediaType) VALUES ('Asimov','Nightfall', 'Book')

我希望能够获得看起来像这样的数据。

Author               Title                     MediaType ItemCount
-------------------- ------------------------- --------- -----------
Heinlein             The Roads Must Roll       CD        2
Heinlein             The Roads Must Roll       Book      2
Heinlein             Time Enough for Love      Book      1
Asimov               The Foundation Trilogy    CD        2
Asimov               The Foundation Trilogy    Book      2
Asimov               I Robot                   CD        1
Asimov               Nightfall                 Book      1

像

这样的东西

UPDATE @myTemp 
SET ItemCount = (SELECT COUNT(1) FROM @myTemp GROUP BY Author, Title)

但是那个子查询返回多行显然不对。

Answer 1

使用sql server，您可以在update语句中使用FROM并将此表链接到自身以获取每个Author，Title实例的计数。

UPDATE @myTemp
SET ItemCount = bookcount
from @myTemp as m
inner join 
(SELECT Author, Title, COUNT(*) as bookcount
 FROM @myTemp
 group by author, title) as mt 
 on m.Author = mt.author and m.Title = mt.Title

不确定我理解你的结果。它更像是一个物品总数。例如，The Roads Must Roll没有2张CD。