如何在SQL Server中将第一行左连接

Question

如何将连接两个表，从第二个表中只选择第一行？

我的问题是一个跟进：我使用的查询 SQL Server: How to Join to first row 在该线程建议。

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，我得到了内连接结果。我想要获得左侧加入结果。所需结果中的tabel1.id应该具有NULL的“2”。怎么做？

Answer 1

使用row_number和left join

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 

OUTPUT：

id category 
1 A 
2 (null) 
3 X 

SQLFIDDLE DEMO

Answer 2

select table1.id, 
(SELECT TOP 1 category FROM table2 WHERE table2.id=table1.id ORDER BY category ASC) AS category 
FROM table1 

Answer 3

SELECT table1.id ,table2.category 
FROM table1 Left join table2 
on table1.id = table2.id 
where table2.category = (select top 1 category from table2 t where table1.id = t.id) 
OR table2.category is NULL