如何模仿LIMIT 1在T-SQL

Question

假設兩個表如下：

insert into tmp1 values(12345) // orderid 
insert into tmp1 values(54321) 
insert into tmp1 values(77777) 

insert into tmp2 values(12345, 1) // orderid, code 
insert into tmp2 values(12345, 2) 
insert into tmp2 values(54321, 1) 
insert into tmp2 values(77777, 1) 
insert into tmp2 values(77777, 2) 
insert into tmp2 values(77777, 3) 

，然後我寫了以下內容：

select a.orderid from tmp1 a 
inner join tmp2 b on b.orderid = a.orderid where b.code in (1,2) 

有沒有一種方法可以讓我剛上車的輸出：

12345, 
54321, 
77777 

這將是一個報告，我只想知道沒有複製的訂單號由於內部連接，雖然內部j oin很重要，因爲有多個代碼可供選擇。

Answer 1

SELECT DISTINCT a.orderid 
FROM tmp1 a 
     INNER JOIN tmp2 b 
       ON b.orderid = a.orderid 
WHERE b.code IN (1, 2) 

Answer 2

select distinct a.orderid from tmp1 a inner join tmp2 b 
on b.orderid = a.orderid where b.code in (1,2) 

Answer 3

您可以使用UNION操作這樣的 -

Select orderid from tmp1 
Union 
Select orderid from tmp2 Where code In (1,2); 

或者你也可以使用內部聯接與鮮明 -

SELECT 
    DISTINCT t1.Orderid 
FROM tmp1 t1 
INNER JOIN tmp2 t2 
ON t1.orderid = t2.orderid 
WHERE t2.code IN (1, 2); 

Answer 4

避免任何連接：

select distinct t2.orderid 
from temp2 t2 
where t2.code in (1,2) 

使用exists（ 「半聯接」）：

select t1.orderid 
from tmp1 t1 
    where exists (
    select null 
    from temp2 t2 
    where t1.orderid = t2.orderid and t2.code in (1,2) 
    ) 

使用窗口函數ROW_NUMBER（）：

select d.* from (
    select t2.orderid 
     , row_number() over(partition by t2.orderid order by t2.code) rn 
    from temp2 t2 
    where t2.code in (1,2) 
    ) d 
where d.rn = 1 

這第三個選項似乎矯枉過正，這個簡單的例子，但該技術是特別適用於查找諸如「第一」或「最後」或「最早」或「最新」等信息。