sql - 如何在 SQL Server 中左连接到第一行

Question

如何左连接两个表，仅从第二个表中选择第一行？

我的问题是以下问题的后续: SQL Server: How to Join to first row我使用了该线程中建议的查询。

CREATE TABLE table1(
  id INT NOT NULL
);
INSERT INTO table1(id) VALUES (1);
INSERT INTO table1(id) VALUES (2);
INSERT INTO table1(id) VALUES (3);
GO

CREATE TABLE table2(
  id INT NOT NULL
, category VARCHAR(1)
);
INSERT INTO table2(id,category) VALUES (1,'A');
INSERT INTO table2(id,category) VALUES (1,'B');
INSERT INTO table2(id,category) VALUES (1,'C');
INSERT INTO table2(id,category) VALUES (3,'X');
INSERT INTO table2(id,category) VALUES (3,'Y');
GO

------------------
SELECT 
table1.* 
,FirstMatch.category
FROM table1

CROSS APPLY (
    SELECT TOP 1 
    table2.id
    ,table2.category   
    FROM table2 
    WHERE table1.id = table2.id
    ORDER BY id
    )
    AS FirstMatch

但是，通过此查询，我得到了内部联接结果。我想要得到左连接结果。所需结果中的 tabel1.id 应为“2”且为 NULL。怎么做？

Answer 1

使用row_number和左连接

with cte as(

select id,
       category,
       row_number() over(partition by id order by category) rn
       from table2
)
select t.id, cte.category
from table1 t
left outer join cte 
on t.id=cte.id and cte.rn=1

输出:

id  category
1   A
2   (null)
3   X

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