SQL：如何添加條件對內部的選擇

Question

1. links: link_id, road 
2. roads: road, code 
2. admin_classes: code, admin_class, priority 

所以，對於一個LINK_ID我有幾個代碼和幾個admin_class +優先原糖 但在結果集我需要爲每個link_id只有一行： link_id和admin_class，那指的是最低優先級。 我儘量讓sql語句，像下面：

select l.link_id, ac.admin_class, min(ac.priority) 
from base.links as l 
inner join base.admin_classes as ac 
on ac.code=l.code 
where ac.admin_class<>0 

但是，首先，它不工作，這是不是我所需要的。我想在結果集只有兩列 - link_id和admin_class

然後我嘗試以下操作：

select l.link_id, (select ac.admin_class from base.admin_classes as ac where ac.code=l.code 
order by ac.priority limit 1) as admin_class from base.links as l 

這是正確的，但我不知道如何在內部選擇添加條件，我只需要admin_class原糖<> 0

我使用PostgreSQL 8.3

預先感謝您。

Answer 1

這不工作？

inner join base.admin_classes as ac 
on ac.code=l.code and ac.admin_class<>0 

Answer 2

我不熟悉PostgreSQL的，但我猜原始查詢不起作用becuse你有沒有group by

然而一個min()聚合功能，下面應該，如果你不工作「T需要min()：

select l.link_id, ac.admin_class 
from base.links as l 
inner join base.admin_classes as ac 
     on ac.code=l.code 
     and ac.admin_class<>0 

否則，請嘗試：

select l.link_id, ac.admin_class, min(ac.priority) 
from base.links as l 
inner join base.admin_classes as ac 
     on ac.code=l.code 
     and ac.admin_class<>0 
group by l.link_id, ac.admin_class 

Answer 3

好吧，我不知道很多關於PostgreSQL的，但這是它是如何與T-SQL來完成：

select l.link_id, ac.admin_class, min(ac.priority) 
from base.links as l 
inner join base.admin_classes as ac 
on ac.code=l.code 
group by l.link_id, ac.admin_class 
having ac.admin_class <> 0 

Answer 4

我相信你應該做兩兩件事。

1日創建一個視圖名爲viewAdminRoads

CREATE VIEW viewAdminRoads 
AS 
SELECT 
roads.code, 
roads.road, 
admin_classes.admin_class, 
admin_classes.priority 
FROM roads 
INNER JOIN admin_classes 
     ON roads.road 
     = admin_classes.road 
WHERE admin_classes.priority <> 0 

建立這些視圖簡化了整個過程

第二屆創建您的選擇

SELECT 
link.link_id, 
MIN(viewAdminRoads.priority) as priority 
FROM links 
INNER JOIN viewAdminRoads 
     ON links.code 
     = viewAdminRoads.code 
GROUP BY link.link_id 

記住，觀點是你的frien d。