更好的sql使用where子句计算项目数

我有这个问题，我知道有更好的方法来写它。下面是一个查询，它对订单进行计数，以找出库存中还剩下什么

DECLARE @reserveDate as Datetime = '10/5/2011 10:20'

SELECT p.Name 
     , p.Quantity
     , (SELECT COUNT(*) 
          FROM [Order] o 
         WHERE o.ProductId = p.Id 
           AND o.Completed = 1) as Completed 
     , (SELECT COUNT(*) 
          FROM [Order] o 
         WHERE o.ProductId = p.Id 
           AND o.Completed <> 1 
           AND o.ModifiedDate >= @reserveDate) as Reserved 
     , (SELECT COUNT(*) 
          FROM [Order] o 
         WHERE o.ProductId = p.Id 
           AND o.Completed <> 1 
           AND o.ModifiedDate < @reserveDate) as ReserveExpired  
   --, (Quantity - Completed - Reserved) as Available
  FROM Product p 

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您可以在T-SQL本身中减去项，例如：

从ID=1的表中选择A-B作为C

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虽然我不确定用子查询实现这一点的最佳方法。我也不知道您是否需要在编程中保留实际数量、已完成、保留和重新服务的Expired值，或者您是否只需要这些值来计算可用数量。如果您需要SQL外部的查询，那么内部的减法将毫无帮助。

您可以使用以下方法替换子查询：

   SELECT p.Name 
        , p.Quantity
        , SUM(CASE WHEN o.completed = 1 THEN 1 ELSE 0 END) AS Completed 
        , SUM(CASE WHEN o.completed <> 1 AND o.ModifiedDate >= @reserveDate THEN 1 ELSE 0 END) AS Reserved 
        , SUM(CASE WHEN o.completed <> 1 AND o.ModifiedDate < @reserveDate THEN 1 ELSE 0 END) AS ReserveExpired  
        , p.Quantity - 
          SUM(CASE WHEN o.completed = 1 THEN 1 ELSE 0 END) -
          SUM(CASE WHEN o.completed <> 1 AND o.ModifiedDate >= @reserveDate THEN 1 ELSE 0 END) AS available
     FROM Product p 
LEFT JOIN ORDER o ON o.productid = p.id
 GROUP BY p.Name, p.Quantity

或者，以下内容等效且易于阅读：

SELECT x.name, 
       x.quantity,
       x.completed,
       x.reserved,
       x.reserveexpired,
       x.quantity - x.completed - x.reserved AS available
  FROM (SELECT p.Name 
             , p.Quantity
             , SUM(CASE WHEN o.completed = 1 THEN 1 ELSE 0 END) AS Completed 
             , SUM(CASE WHEN o.completed <> 1 AND o.ModifiedDate >= @reserveDate THEN 1 ELSE 0 END) AS Reserved 
             , SUM(CASE WHEN o.completed <> 1 AND o.ModifiedDate < @reserveDate THEN 1 ELSE 0 END) AS ReserveExpired  
          FROM Product p 
     LEFT JOIN ORDER o ON o.productid = p.id
      GROUP BY p.Name, p.Quantity) x

可以使用交叉应用/外部应用运算符：

CREATE INDEX aaa
ON [Order](ProductId)
INCLUDE (Completed,ModifiedDate);

PRINT '***** Sol1 *****'
SELECT 
     p.Name 
    ,p.Quantity
    ,ISNULL(ca.Completed,0) Completed
    ,ISNULL(ca.Reserved,0) Reserved
    ,ISNULL(ca.ReserveExpired,0) ReserveExpired
    ,p.Quantity - ISNULL(ca.Completed,0) - ISNULL(ca.Reserved,0) Available
FROM Product p 
OUTER APPLY
(
    SELECT 
         SUM(CASE WHEN o.Completed = 1 THEN 1 ELSE 0 END) Completed
        ,SUM(CASE WHEN o.Completed <> 1 AND o.ModifiedDate >= @reserveDate THEN 1 ELSE 0 END) Reserved
        ,SUM(CASE WHEN o.Completed <> 1 AND o.ModifiedDate < @reserveDate THEN 1 ELSE 0 END) ReserveExpired
    FROM [Order] o --WITH(FORCESEEK) or WITH(INDEX=aaa)
    WHERE o.ProductId = p.Id 
) ca;

PRINT '***** Sol2 *****'
SELECT 
     p.Name 
    ,p.Quantity
    ,ISNULL(q.Completed,0) Completed
    ,ISNULL(q.Reserved,0) Reserved
    ,ISNULL(q.ReserveExpired,0) ReserveExpired
    ,p.Quantity - ISNULL(q.Completed,0) - ISNULL(q.Reserved,0) Available
FROM    Product p
LEFT MERGE JOIN --or LEFT JOIN
(
    SELECT   o.ProductId
            ,SUM(CASE WHEN o.Completed = 1 THEN 1 ELSE 0 END) Completed
            ,SUM(CASE WHEN o.Completed <> 1 AND o.ModifiedDate >= @reserveDate THEN 1 ELSE 0 END) Reserved
            ,SUM(CASE WHEN o.Completed <> 1 AND o.ModifiedDate < @reserveDate THEN 1 ELSE 0 END) ReserveExpired     
    FROM    [Order] o 
    GROUP BY o.ProductId
) q ON p.Id = q.ProductId;

CREATE INDEX aaa
ON [Order](ProductId)
INCLUDE (Completed,ModifiedDate);

PRINT '***** Sol1 *****'
SELECT 
     p.Name 
    ,p.Quantity
    ,ISNULL(ca.Completed,0) Completed
    ,ISNULL(ca.Reserved,0) Reserved
    ,ISNULL(ca.ReserveExpired,0) ReserveExpired
    ,p.Quantity - ISNULL(ca.Completed,0) - ISNULL(ca.Reserved,0) Available
FROM Product p 
OUTER APPLY
(
    SELECT 
         SUM(CASE WHEN o.Completed = 1 THEN 1 ELSE 0 END) Completed
        ,SUM(CASE WHEN o.Completed <> 1 AND o.ModifiedDate >= @reserveDate THEN 1 ELSE 0 END) Reserved
        ,SUM(CASE WHEN o.Completed <> 1 AND o.ModifiedDate < @reserveDate THEN 1 ELSE 0 END) ReserveExpired
    FROM [Order] o --WITH(FORCESEEK) or WITH(INDEX=aaa)
    WHERE o.ProductId = p.Id 
) ca;

PRINT '***** Sol2 *****'
SELECT 
     p.Name 
    ,p.Quantity
    ,ISNULL(q.Completed,0) Completed
    ,ISNULL(q.Reserved,0) Reserved
    ,ISNULL(q.ReserveExpired,0) ReserveExpired
    ,p.Quantity - ISNULL(q.Completed,0) - ISNULL(q.Reserved,0) Available
FROM    Product p
LEFT MERGE JOIN --or LEFT JOIN
(
    SELECT   o.ProductId
            ,SUM(CASE WHEN o.Completed = 1 THEN 1 ELSE 0 END) Completed
            ,SUM(CASE WHEN o.Completed <> 1 AND o.ModifiedDate >= @reserveDate THEN 1 ELSE 0 END) Reserved
            ,SUM(CASE WHEN o.Completed <> 1 AND o.ModifiedDate < @reserveDate THEN 1 ELSE 0 END) ReserveExpired     
    FROM    [Order] o 
    GROUP BY o.ProductId
) q ON p.Id = q.ProductId;