[英]How to join two tables in a one-to-many relation, and keep only one-to-one records?
I have two tables : Folders
and References
. 我有两个表:
Folders
和References
。 A folder can have 0 to 2 references (usually 1 or 2). 一个文件夹可以有0到2个引用(通常为1或2)。
I have an existing query that search Folders
that respect certains conditions. 我有一个现有的查询,搜索尊重某些条件的
Folders
。 The query must have a new condition : Find only Folders
with one reference. 查询必须具有新条件:仅查找具有一个引用的
Folders
。
Ideally , to limits change to this old application, the new condition should be within the WHERE
clause only (This rule out group by x, having y
). 理想情况下 ,为了限制对此旧应用程序的更改,新条件应仅在
WHERE
子句内(此规则group by x, having y
排除group by x, having y
)。
Ex : 例如:
FolderId Name
0 Folder0
1 Folder1
2 Folder2
RefId FolderId Name
0 1 ref1
1 2 ref2
2 2 ref3
The output should only contain the FolderId 1 输出应该只包含FolderId 1
Use the query below to create a temporary table or create a table variable: 使用以下查询创建临时表或创建表变量:
create table #Temp
(
column_name int
)
insert into #Temp
SELECT column_name
FROM table_name
GROUP BY column_name
HAVING COUNT(column_name) = 1;
Then use the temp table with join
to other tables and place the conditions you want. 然后使用与临时表
join
到其他表,并把你想要的条件。
You probably want this: 你可能想要这个:
select . . .
from folders f join
(select r.*, count(*) over (partition by folderid) as cnt
from references r
) r
on f.folderid = r.folderid and cnt = 1;
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