SELECT 没有 FROM<table> 但有 APPLY / JOIN

Question

我写了一个程序，需要几个外键ID来做进一步的操作

表md_machines

+----+-------+
| id | name  |
+----+-------+
| 1  | c432  |
| 2  | c431  |
| n  | ...   |
+----+-------+

我在我的程序中的查询：

SELECT TOP 1  
@m1 = m1.id,  
@m2 = m2.id,  
@m3 = m3.id,  
@m4 = m4.id  
FROM md_machines  
OUTER APPLY(SELECT TOP 1 id FROM md_machines WHERE name = @p1) m1  
OUTER APPLY(SELECT TOP 1 id FROM md_machines WHERE name = @p2) m2  
OUTER APPLY(SELECT TOP 1 id FROM md_machines WHERE name = @p3) m3  
OUTER APPLY(SELECT TOP 1 id FROM md_machines WHERE name = @p4) m4  

一切正常，但md_machines 有超过 300k 行。当我用空表替换 FROM md_machines 时，这个查询大约快 10%。
我怎样才能避免FROM md_machines？对我来说没有必要。

Answer 1

使用简单的变量赋值：

SET @m1 = (SELECT TOP 1 id FROM md_machines WHERE name = @p1);
SET @m2 = (SELECT TOP 1 id FROM md_machines WHERE name = @p2);
SET @m3 = (SELECT TOP 1 id FROM md_machines WHERE name = @p3);
SET @m4 = (SELECT TOP 1 id FROM md_machines WHERE name = @p4);

TOP 1 没有明确的ORDER BY 是不稳定的。

Answer 2

选择一次

SELECT name, min(id) 
FROM md_machines 
WHERE (name = @p1 or name = @p2 or name = @p3 or name = @p4)
group by name

然后使用WITH上面的查询

WITH sub (name, id)  
AS  
(  
    the suquery  
)

SELECT TOP 1
@m1 = m1.id,
@m2 = m2.id,
@m3 = m3.id,
@m4 = m4.id
FROM (SELECT TOP 1 id FROM sub WHERE name = @p1) m1,
     (SELECT TOP 1 id FROM sub WHERE name = @p2) m2,
     (SELECT TOP 1 id FROM sub WHERE name = @p3) m3,
     (SELECT TOP 1 id FROM sub WHERE name = @p4) m4

顺便说一句：在name 列上有索引会很好

更新：我认为这个也可以

SELECT TOP 1
@m1 = m1.id,
@m2 = m2.id,
@m3 = m3.id,
@m4 = m4.id
FROM (SELECT TOP 1 id FROM md_machines WHERE name = @p1) m1,
     (SELECT TOP 1 id FROM md_machines WHERE name = @p2) m2,
     (SELECT TOP 1 id FROM md_machines WHERE name = @p3) m3,
     (SELECT TOP 1 id FROM md_machines WHERE name = @p4) m4