SQL：根据另一个表中多行的值选择一个值

Question

我有一个表 NETWORKS，其中每个网络可以有多个 CIRCUITS。每个网络都有一个整体状态（红色/黄色/绿色），每个电路都有一个单独的状态（红色/绿色）。每个电路的状态都是手动设置的。网络状态如下：

• 如果它的所有电路都是绿色的 --> 绿色
• 如果它的所有电路都是红色的 --> 红色
• 如果至少 1 个（但不是全部）电路为绿色 --> 黄色
• 如果没有电路 --> NULL（无状态）

我正在尝试选择所有网络，它们的状态由 SELECT 动态确定，而不必将状态作为表中的一列保存和管理。我想不出一种有效的方法来做到这一点。我现在的工作（都是小表，

SELECT
(CASE 
    WHEN (
        SELECT COUNT(*) 
        FROM NETWORK_CIRCUITS 
        WHERE network_id = N.network_id
    ) = 0 THEN 'noStatus'
    WHEN (
        SELECT COUNT(*) 
        FROM NETWORK_CIRCUITS 
        WHERE network_id = N.network_id
        AND [status] = 'greenStatus'
    ) = (
        SELECT COUNT(*) 
        FROM NETWORK_CIRCUITS 
        WHERE network_id = SSN.network_id
    ) THEN 'greenStatus'
    WHEN (
        SELECT COUNT(*) 
        FROM NETWORK_CIRCUITS 
        WHERE network_id = N.network_id
        AND [status] = 'redStatus'
    ) = (
        SELECT COUNT(*) 
        FROM NETWORK_CIRCUITS 
        WHERE network_id = N.network_id
    ) THEN 'redStatus'
    ELSE 'yellowStatus'
END) network_status
FROM NETWORKS N

Answer 1

这是一种方法：

SELECT
    CASE 
        WHEN CircuitCount IS NULL      THEN 'noStatus'
        WHEN GreenCount = CircuitCount THEN 'greenStatus'
        WHEN GreenCount = 0            THEN 'redStatus'
        ELSE 'yellowStatus'
    END As network_status
FROM NETWORKS As N
LEFT JOIN
(   SELECT  COUNT(*) As CircuitCount,
            COUNT(NULLIF([status],'redStatus')) As GreenCount,
            network_id
    FROM    NETWORK_CIRCUITS
    GROUP BY network_id
)   As C    ON N.network_id = C.network_id

Answer 2

我的第一个想法是：

select network_id, case when green=0 and red=0 then null when green=0 then 'red' else when red=0 then 'green' else 'yellow' end as status
from
(select n.network_id,
  sum(case when c.status='green' then 1 else 0 end) as green,
  sum(case when c.status='red' then 1 else 0 end) as red
from network n
join circuit c on c.network_id=n.network_id
group by n.network_id)

Answer 3

我认为这应该可以满足您的需求。它也应该比使用一堆子查询更快。

SELECT CASE
         WHEN circuits.network_id is NULL THEN 'No Status'
         WHEN circuits.greenCount = circuits.totalCircuits THEN 'Green'
         WHEN circuits.greenCount >= 1 and circuits.redCount >= 1 THEN 'Yellow'
         WHEN circuits.redCount = circuits.totalCircuits THEN 'Red'
       END as network_status
      , N.network_id
FROM NETWORKS N
LEFT JOIN 
    (SELECT network_id
           , sum(CASE WHEN [status] = 'redStatus' THEN 1 ELSE 0 END) as redCount
           , sum(CASE WHEN [status] = 'greenStatus' THEN 1 ELSE 0 END) as greenCount
           , count(*) as totalCircuits
    FROM NETWORK_CIRCUITS
    GROUP BY network_id) as circuits ON circuits.network_id = N.network_id