How to get set-based solution for a difficult join - sql

Question

I've got a Table with a list of Products.
I've got a Table with a list of Merchants.
I've got a Table with a list of Warehouses.
I've got a WarehouseMerchant Table so a a warehouse can have many merchants assigned.
I've got a WarehouseMerchantProduct Table so a merchant can have its products listed in a warehouse
I've got a Merchant Link Table which can link 2 merchants
MerchantLinkID int
MasterMerchantID int
LinkedMerchantID int
PreferenceOrder int
What i need to do is pass in a product ID and get a unique list of merchants products who can sell it.
4 Rules
If the merchant has a linked merchant who also sells the product, then don't show the linked merchants product
If the merchant has a linked merchant who also sells the product, but the master merchants stock level is 0, then only show the linked merchants product
In the event that a merchant has stock in multiple warehouses, then there is a preference order in the warehouseMerchant Table to select which one to use
There should only be one product per merchant after this has all ran
I've been racking my brains trying to figure out a solution. I was going down the route of cursors, but it was getting a bit unwieldy. Then i was given the 4 rules as a requirement which sent me back to the drawing board.
If anyone can think of a quick and easy(ish) set-based solution for this then I'd love to hear it.
Here is the script to create the tables and relationships
CREATE TABLE [dbo].[MW_Merchant](
[MerchantID] [int] IDENTITY(1,1) NOT NULL,
[MerchantName] [nvarchar](150) NOT NULL,
[MerchantCode] [char](10) NOT NULL,
[Active] [bit] NOT NULL,
CONSTRAINT [PK_Merchant] PRIMARY KEY CLUSTERED
(
[MerchantID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY],
CONSTRAINT [ucMerchantCode] UNIQUE NONCLUSTERED
(
[MerchantCode] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
SET ANSI_PADDING OFF
GO
/****** Object: Table [dbo].[MW_Warehouse] Script Date: 04/30/2013 09:17:39 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
SET ANSI_PADDING ON
GO
CREATE TABLE [dbo].[MW_Warehouse](
[WarehouseID] [int] IDENTITY(1,1) NOT NULL,
[WarehouseName] [nvarchar](150) NOT NULL,
[WarehouseCode] [varchar](10) NOT NULL,
[Active] [bit] NOT NULL,
CONSTRAINT [PK_Warehouse] PRIMARY KEY CLUSTERED
(
[WarehouseID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
SET ANSI_PADDING OFF
GO
/****** Object: Table [dbo].[MW_WarehouseMerchantProduct] Script Date: 04/30/2013 09:17:39 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
SET ANSI_PADDING ON
GO
CREATE TABLE [dbo].[MW_WarehouseMerchantProduct](
[WarehouseMerchantProductID] [int] IDENTITY(1,1) NOT NULL,
[WarehouseID] [int] NOT NULL,
[MerchantID] [int] NOT NULL,
[ProductCode] [varchar](30) NOT NULL,
[StockLevel] [int] NOT NULL,
[Active] [bit] NOT NULL,
[PriorityOrder] [int] NOT NULL,
CONSTRAINT [PK_MW_WarehouseProduct] PRIMARY KEY CLUSTERED
(
[WarehouseMerchantProductID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
SET ANSI_PADDING OFF
GO
/****** Object: Table [dbo].[MW_WarehouseMerchant] Script Date: 04/30/2013 09:17:39 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[MW_WarehouseMerchant](
[WarehouseMerchantID] [int] IDENTITY(1,1) NOT NULL,
[WarehouseID] [int] NOT NULL,
[MerchantID] [int] NOT NULL,
[Active] [bit] NOT NULL,
[PreferenceOrder] [int] NOT NULL,
CONSTRAINT [PK_MW_WarehouseMerchant] PRIMARY KEY CLUSTERED
(
[WarehouseMerchantID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
/****** Object: Table [dbo].[MW_MerchantLink] Script Date: 04/30/2013 09:17:39 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[MW_MerchantLink](
[MerchantLinkID] [int] IDENTITY(1,1) NOT NULL,
[MasterMerchantID] [int] NOT NULL,
[LinkedMerchantID] [int] NOT NULL,
[PreferenceOrder] [int] NOT NULL,
CONSTRAINT [PK_MW_MerchantLink] PRIMARY KEY CLUSTERED
(
[MerchantLinkID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
/****** Object: Default [DF_MW_WarehouseMerchant_PreferenceOrder] Script Date: 04/30/2013 09:17:39 ******/
ALTER TABLE [dbo].[MW_WarehouseMerchant] ADD CONSTRAINT [DF_MW_WarehouseMerchant_PreferenceOrder] DEFAULT ((10)) FOR [PreferenceOrder]
GO
/****** Object: Default [DF_MW_WarehouseMerchantProduct_PriorityOrder] Script Date: 04/30/2013 09:17:39 ******/
ALTER TABLE [dbo].[MW_WarehouseMerchantProduct] ADD CONSTRAINT [DF_MW_WarehouseMerchantProduct_PriorityOrder] DEFAULT ((10)) FOR [PriorityOrder]
GO
/****** Object: ForeignKey [FK_MW_MerchantLink_MW_Merchant] Script Date: 04/30/2013 09:17:39 ******/
ALTER TABLE [dbo].[MW_MerchantLink] WITH CHECK ADD CONSTRAINT [FK_MW_MerchantLink_MW_Merchant] FOREIGN KEY([MasterMerchantID])
REFERENCES [dbo].[MW_Merchant] ([MerchantID])
GO
ALTER TABLE [dbo].[MW_MerchantLink] CHECK CONSTRAINT [FK_MW_MerchantLink_MW_Merchant]
GO
/****** Object: ForeignKey [FK_MW_MerchantLink_MW_Merchant1] Script Date: 04/30/2013 09:17:39 ******/
ALTER TABLE [dbo].[MW_MerchantLink] WITH CHECK ADD CONSTRAINT [FK_MW_MerchantLink_MW_Merchant1] FOREIGN KEY([LinkedMerchantID])
REFERENCES [dbo].[MW_Merchant] ([MerchantID])
GO
ALTER TABLE [dbo].[MW_MerchantLink] CHECK CONSTRAINT [FK_MW_MerchantLink_MW_Merchant1]
GO
/****** Object: ForeignKey [FK_MW_WarehouseMerchant_MW_Merchant] Script Date: 04/30/2013 09:17:39 ******/
ALTER TABLE [dbo].[MW_WarehouseMerchant] WITH CHECK ADD CONSTRAINT [FK_MW_WarehouseMerchant_MW_Merchant] FOREIGN KEY([MerchantID])
REFERENCES [dbo].[MW_Merchant] ([MerchantID])
GO
ALTER TABLE [dbo].[MW_WarehouseMerchant] CHECK CONSTRAINT [FK_MW_WarehouseMerchant_MW_Merchant]
GO
/****** Object: ForeignKey [FK_MW_WarehouseMerchant_MW_Warehouse] Script Date: 04/30/2013 09:17:39 ******/
ALTER TABLE [dbo].[MW_WarehouseMerchant] WITH CHECK ADD CONSTRAINT [FK_MW_WarehouseMerchant_MW_Warehouse] FOREIGN KEY([WarehouseID])
REFERENCES [dbo].[MW_Warehouse] ([WarehouseID])
GO
ALTER TABLE [dbo].[MW_WarehouseMerchant] CHECK CONSTRAINT [FK_MW_WarehouseMerchant_MW_Warehouse]
GO
/****** Object: ForeignKey [FK_MW_WarehouseMerchantProduct_MW_Merchant] Script Date: 04/30/2013 09:17:39 ******/
ALTER TABLE [dbo].[MW_WarehouseMerchantProduct] WITH CHECK ADD CONSTRAINT [FK_MW_WarehouseMerchantProduct_MW_Merchant] FOREIGN KEY([MerchantID])
REFERENCES [dbo].[MW_Merchant] ([MerchantID])
GO
ALTER TABLE [dbo].[MW_WarehouseMerchantProduct] CHECK CONSTRAINT [FK_MW_WarehouseMerchantProduct_MW_Merchant]
GO
/****** Object: ForeignKey [FK_MW_WarehouseProduct_MW_Warehouse] Script Date: 04/30/2013 09:17:39 ******/
ALTER TABLE [dbo].[MW_WarehouseMerchantProduct] WITH CHECK ADD CONSTRAINT [FK_MW_WarehouseProduct_MW_Warehouse] FOREIGN KEY([WarehouseID])
REFERENCES [dbo].[MW_Warehouse] ([WarehouseID])
GO
ALTER TABLE [dbo].[MW_WarehouseMerchantProduct] CHECK CONSTRAINT [FK_MW_WarehouseProduct_MW_Warehouse]
GO
And some test data
/****** Object: Table [dbo].[MW_Merchant] Script Date: 04/30/2013 09:41:50 ******/
SET IDENTITY_INSERT [dbo].[MW_Merchant] ON
INSERT [dbo].[MW_Merchant] ([MerchantID], [MerchantName], [MerchantCode], [Active]) VALUES (24, N'Merchant 1', N'MERCH1 ', 1)
INSERT [dbo].[MW_Merchant] ([MerchantID], [MerchantName], [MerchantCode], [Active]) VALUES (27, N'Merchant 2', N'MERCH2 ', 1)
INSERT [dbo].[MW_Merchant] ([MerchantID], [MerchantName], [MerchantCode], [Active]) VALUES (28, N'Merchant 3', N'MERCH3 ', 1)
SET IDENTITY_INSERT [dbo].[MW_Merchant] OFF
/****** Object: Table [dbo].[MW_Warehouse] Script Date: 04/30/2013 09:41:50 ******/
SET IDENTITY_INSERT [dbo].[MW_Warehouse] ON
INSERT [dbo].[MW_Warehouse] ([WarehouseID], [WarehouseName], [WarehouseCode], [Active]) VALUES (12, N'Warehouse 1', N'WARE1', 1)
INSERT [dbo].[MW_Warehouse] ([WarehouseID], [WarehouseName], [WarehouseCode], [Active]) VALUES (13, N'Warehouse 2', N'WARE2', 1)
INSERT [dbo].[MW_Warehouse] ([WarehouseID], [WarehouseName], [WarehouseCode], [Active]) VALUES (14, N'Warehouse 3', N'WARE3', 1)
SET IDENTITY_INSERT [dbo].[MW_Warehouse] OFF
/****** Object: Table [dbo].[MW_WarehouseMerchantProduct] Script Date: 04/30/2013 09:41:50 ******/
SET IDENTITY_INSERT [dbo].[MW_WarehouseMerchantProduct] ON
INSERT [dbo].[MW_WarehouseMerchantProduct] ([WarehouseMerchantProductID], [WarehouseID], [MerchantID], [ProductCode], [StockLevel], [Active], [PriorityOrder]) VALUES (93, 13, 24, N'TESTPRODUCT', 20, 1, 10)
INSERT [dbo].[MW_WarehouseMerchantProduct] ([WarehouseMerchantProductID], [WarehouseID], [MerchantID], [ProductCode], [StockLevel], [Active], [PriorityOrder]) VALUES (96, 14, 24, N'TESTPRODUCT', 20, 1, 10)
INSERT [dbo].[MW_WarehouseMerchantProduct] ([WarehouseMerchantProductID], [WarehouseID], [MerchantID], [ProductCode], [StockLevel], [Active], [PriorityOrder]) VALUES (97, 14, 27, N'TESTPRODUCT', 10, 1, 10)
SET IDENTITY_INSERT [dbo].[MW_WarehouseMerchantProduct] OFF
/****** Object: Table [dbo].[MW_MerchantLink] Script Date: 04/30/2013 09:41:50 ******/
SET IDENTITY_INSERT [dbo].[MW_MerchantLink] ON
INSERT [dbo].[MW_MerchantLink] ([MerchantLinkID], [MasterMerchantID], [LinkedMerchantID], [PreferenceOrder]) VALUES (22, 24, 27, 1)
INSERT [dbo].[MW_MerchantLink] ([MerchantLinkID], [MasterMerchantID], [LinkedMerchantID], [PreferenceOrder]) VALUES (23, 24, 28, 2)
SET IDENTITY_INSERT [dbo].[MW_MerchantLink] OFF
In the example data, my returned row should be the WarehouseMerchantProductID 93
93,96 and 97 all have stock, however
93 and 96 belong to the same merchant and should be selected based on PreferenceOrder in the WarehouseMerchant Table
Merchant 27 is linked to 24 so shouldnt appear (unless 24 goes out of stock)

Answer 1

Note, the following query is written for Oracle and assumes that a LinkedMerchant only has 1 MasterMerchant. You can play around with the code here. Also note that I have included more test data than you provided in order to test some additional cases.
SELECT MERCHANTID, WAREHOUSEID, PRODUCTCODE
FROM (
SELECT 0
, WMP.MERCHANTID
, WMP.WAREHOUSEID
, WMP.PRODUCTCODE
, RANK() OVER (PARTITION BY ML.MASTERMERCHANTID, WMP.PRODUCTCODE ORDER BY ML.PREFERENCEORDER ASC NULLS LAST) MERCH_RANK
, RANK() OVER (PARTITION BY WMP.MERCHANTID, WMP.PRODUCTCODE ORDER BY WM.PREFERENCEORDER ASC NULLS LAST) WARE_RANK
FROM MW_WAREHOUSEMERCHANTPRODUCT WMP
LEFT OUTER JOIN MW_WAREHOUSEMERCHANT WM ON 0=0
AND WMP.MERCHANTID = WM.MERCHANTID
AND WMP.WAREHOUSEID = WM.WAREHOUSEID
LEFT OUTER JOIN MW_MERCHANTLINK ML ON 0=0
AND WMP.MERCHANTID = ML.LINKEDMERCHANTID
LEFT OUTER JOIN MW_WAREHOUSEMERCHANTPRODUCT MASTER ON 0=0
AND ML.MASTERMERCHANTID = MASTER.MERCHANTID
AND WMP.PRODUCTCODE = MASTER.PRODUCTCODE
WHERE 0=0
AND WMP.STOCKLEVEL > 0
AND NVL(MASTER.STOCKLEVEL, 0) <= 0
)
WHERE 0=0
AND MERCH_RANK = 1
AND WARE_RANK = 1
;