Question

I have this query and I know there is a better way to write it. Here is the query which counts the orders to find out what is left in Inventory.

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 

====================================

here is a script for the tables

IF EXISTS ( SELECT  * FROM  sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[Order]') AND type IN ( N'U' ) )    DROP TABLE [Order]
IF EXISTS ( SELECT  * FROM  sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[Product]') AND type IN ( N'U' ) )  DROP TABLE [Product]

-- Product --
PRINT N' [Product]  ' 
CREATE TABLE Product
    (
      [Id] INT NOT NULL IDENTITY PRIMARY KEY,
      [Name] NVARCHAR(50) ,
      [Quantity] INT ,
    ); 
SET IDENTITY_INSERT Product ON 
INSERT  INTO Product ( [Id] , [Name] ,  [Quantity]) VALUES  ( '1', 'Tea Package',  7000 )
INSERT  INTO Product ( [Id] , [Name] ,  [Quantity]) VALUES  ( '2', 'Sugar Package',  8000)
SET IDENTITY_INSERT Product OFF 



-- Order --
PRINT N' [Order]'

CREATE TABLE [Order]
    (
      [Id] INT NOT NULL IDENTITY PRIMARY KEY ,
      [ProductId] INT   ,
      [Completed] Bit,
      [ModifiedDate] DATETIME 
    ); 
ALTER TABLE [Order] ADD CONSTRAINT FK_Product_Order FOREIGN KEY (ProductId) REFERENCES [Product] (Id) 
GO  
INSERT  INTO [Order] ([ProductId],  [Completed],   [ModifiedDate]  ) VALUES  ( 1, 1,   '10/5/2011 10:10'  )  
INSERT  INTO [Order] ([ProductId],  [Completed],   [ModifiedDate]  ) VALUES  ( 1, 1,   '10/5/2011 10:10'  )  
INSERT  INTO [Order] ([ProductId],  [Completed],   [ModifiedDate]  ) VALUES  ( 1, 0,   '10/5/2011 10:10'  )  
INSERT  INTO [Order] ([ProductId],  [Completed],   [ModifiedDate]  ) VALUES  ( 1, 0,   '10/5/2011 10:10'  )  
INSERT  INTO [Order] ([ProductId],  [Completed],   [ModifiedDate]  ) VALUES  ( 1, 0,   '10/5/2011 11:10'  )  
INSERT  INTO [Order] ([ProductId],  [Completed],   [ModifiedDate]  ) VALUES  ( 1, 0,   '10/6/2011 11:10'  )  
INSERT  INTO [Order] ([ProductId],  [Completed],   [ModifiedDate]  ) VALUES  ( 1, 0,   '10/6/2011 11:10'  )  
INSERT  INTO [Order] ([ProductId],  [Completed],   [ModifiedDate]  ) VALUES  ( 2, 1,   '10/5/2011 10:10'  )  
INSERT  INTO [Order] ([ProductId],  [Completed],   [ModifiedDate]  ) VALUES  ( 2, 1,   '10/5/2011 10:10'  )  
INSERT  INTO [Order] ([ProductId],  [Completed],   [ModifiedDate]  ) VALUES  ( 2, 0,   '10/5/2011 10:10'  )  
INSERT  INTO [Order] ([ProductId],  [Completed],   [ModifiedDate]  ) VALUES  ( 2, 0,   '10/6/2011 10:10'  )  

Answer 1

You can replace the subqueries using:

   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

Alternately, the following is equivalent & easier to read:

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