来自多个表的 SQL 数据

Question

I have four tables setup as follows

Products

• ProductCode
• ProductDescription
• ProductCategory

BranchData

• ProductCode
• BranchID
• BranchCosting

BranchCostings

• BranchCosting
• ListPrice
• CostPrice

I am trying to create a query which will display a table as follows

• ProductCode
• ProductDescription
• Branch1Cost
• Branch2Cost

with the following where clauses

• ProductCategory = 102,
• Branch2Cost != Branch1Cost

I have very limited SQL knowledge so am wondering how I would write a select query to select this data, I can get a table to display data from the product table and the Branch Data table but I am unable to get the Cost price for both branches from the Branch Costings table.

The SQL I am trying is as follows

Select P.ProductCode, P.ProductDescription, BC.CostPrice as B1Cost,         BC.CostPrice as B2Cost
From  Products P, BranchData BD, BranchCostings BC
Left Join BranchData on BC.BranchCosting = BD.BranchCosting
Where BD.BranchID = Branch1

I have been researching joins but this is where I am stuck, I did not know that you could join a table more than once and I am unsure which join to use (I think its a left join)

I have Just tried using the following which gives me all products in category 102 but it gives me these products for each cost price

Select P.ProductCode, P.ProductDescription, BC.CostPrice as B1Cost, BC.CostPrice as B2Cost
From  Products P, BranchCostings BC
Left Join Products as B0 on B0.BranchCosting = BC.BranchCosting
Left Join BranchData as B1 on B1.BranchCosting = BC.BranchCosting
Left Join BranchData as B2 on B2.BranchCosting = BC.BranchCosting
Where P.C_CATEGORY = 102

I now have the following which displays all the products in the category in the where statement but the B1CostPrice and the B2CostPrice are showing as NULL

Select ProductCode, ProductDescription, B1.CostPrice, B2.CostPrice
From Products
left Join BranchData as B0 on Products.ProductCode = B0.ProductCode
left Join BranchCostings as B1 on B0.BranchCosting = B1.BranchCosting
left Join BranchCostings as B2 on B0.BranchCosting = B2.BranchCosting
Where C_CATEGORY = 102

I have tried changing the join types to see if this would display correctly but it either shows no data at all or shows the correct products with the Cost Price columns as NULL

Data Structure and expected table

Products

ProductCode ProductDescription  Category
-----------------------------------------
Product1    Product Description 1       102
Product2    Product Description 2     102
Product3    Product Description 3     102
Product4    Product Description 4     102
Product5    Product Description 5     102
Product6    Product Description 6        99

BranchData

Product Code    BranchID     BranchCosting
Product1        B1           1
Product1        B2           2
Product2        B1           3
Product2        B2           4
Product3        B1           5
Product3        B2           6
Product4        B1           7
Product4        B2           8
Product5        B1           9
Product5        B2           10
Product6        B1           11
Product6        B2           12

BranchCostings

 BranchCosting  ListPrice   CostPrice
 1              2           1
 2              4           1
 3              6           3
 4              8           4
 5              10          5
 6              12          6
 7              14          7
 8              16          7
 9              18          9
 10             20          10
 11             22          11
 12             24          12

Expected Table

ProductCode     ProductDescription         B1CostPrice  B2CostPrice
Product2        Product Description 2      3              4
Product3        Product Description 3      5              6
Product5        Product Description 5      9             10
Product6        Product Description 6      11            12

In the above table Product1 and Product 4 are not shown as the CostPrice is the same for both B1 and B2

Answer 1

Ok I have finally figured this out.

The select statement I used is as below

SELECT ProductCode, ProductDescription, B1.CostPrice as B1CostPrice, B2CostPrice as B2CostPrice
FROM Products
left JOIN BranchData as B0 on Products.ProductCode = B0.ProductCode
left JOIN BranchData as B9 on Products.ProductCode= B9.ProductCode
left join BranchCostings as B1 on B0.BranchCosting = B1.BranchCosting
left join BranchCostings as B2 on B9.BranchCosting = B2.BranchCosting
Where B0.Branch = Branch1 AND B9.Branch = Branch2 AND Products.Category = 102