从两个不同的表为视图创建一列（SQL Server）

Question

I have three tables A, B and C in a SQL Server, B and C share the same foreign key ( AgencyID ) from A , and they also share one same column name: AgencyName

Table A : AgencyID , AgencyDate ,

Table B : AgencyID , AgencyNumber , AgencyName

Table C : AgencyID , AgencyInvoiceNumber , AgencyName

To help you understand the relationship, basically these 3 tables are generated with Entity Framework , where B and C each extends A .

Now I am trying to create one View from these three tables in the following way:

CREATE VIEW [rdo].[Agencies] AS
SELECT
    A_1.AgencyID, A_1.AgencyDate,
    dbo.B.AgencyNumber, dbo.B.AgencyName,
    dbo.C.AgencyInvoiceNumber, dbo.C.AgencyName
FROM dbo.A AS A_1 LEFT OUTER JOIN
        dbo.B ON A_1.AgencyID = dbo.B.AgencyID LEFT OUTER JOIN
        dbo.C ON A_1.AgencyID = dbo.C.AgencyID

Now, apparently the script errors about AgencyName that column name in a view should be unique :( How would I go about this so that both AgencyName values from B and C are merged into one view?

Note: I know the example isn't perfect as one could just move the column to table A, but it's just a simplified example, in reality I really cannot simply move the column :(

---

Reason for having to have duplicated columns in B and C:

Here's the relationships:

            A 
           / \
          A1  A2
         /      \
        B        C

A1 and A2 extends A, B and C extends A1 and A2 respectively. AgencyName is an unique attribute that only B and C has. I know a refactoring could be done here to try to get rid of this weird hierarchy, but this legacy project has already grown too big and I cannot afford the cost atm.

Answer 1

Give different names to those columns in the view whose name are same in your tables like below

CREATE VIEW [rdo].[Agencies] AS
SELECT
    A_1.AgencyID, A_1.AgencyDate,
    dbo.B.AgencyNumber, dbo.B.AgencyName AS AgencyName_B,
    dbo.C.AgencyInvoiceNumber, dbo.C.AgencyName AS AgencyName_C
FROM dbo.A AS A_1 LEFT OUTER JOIN
        dbo.B ON A_1.AgencyID = dbo.B.AgencyID LEFT OUTER JOIN
        dbo.C ON A_1.AgencyID = dbo.C.AgencyID

Answer 2

CREATE VIEW [rdo].[Agencies] AS
SELECT
    A_1.AgencyID, A_1.AgencyDate,
    dbo.B.AgencyNumber, dbo.B.AgencyName as AgencyNameB,
    dbo.C.AgencyInvoiceNumber, dbo.C.AgencyName as AgencyNameC,
FROM dbo.A AS A_1 LEFT OUTER JOIN
        dbo.B ON A_1.AgencyID = dbo.B.AgencyID LEFT OUTER JOIN
        dbo.C ON A_1.AgencyID = dbo.C.AgencyID

give alice name to AgencyName column. In your case the table dbo.table.AgencyName will have a column called 'AgencyName' for Tables B and C . That is the problem. Give alicename to any one of the column or both.

Answer 3

Try using COALESCE to pick the first non null AgencyName:

CREATE VIEW [rdo].[Agencies] AS
SELECT
    A_1.AgencyID, A_1.AgencyDate,
    dbo.B.AgencyNumber, 
    COALESCE(dbo.B.AgencyName, dbo.C.AgencyName) As AgencyName,
    dbo.C.AgencyInvoiceNumber 
FROM dbo.A AS A_1 LEFT OUTER JOIN
        dbo.B ON A_1.AgencyID = dbo.B.AgencyID LEFT OUTER JOIN
        dbo.C ON A_1.AgencyID = dbo.C.AgencyID

You may face the issue where AgencyName is different in B & C in which case you will need a set of rules to follow. My recommendation is to only have AgencyName in one of the tables (Table A?) otherwise you face potential update discrepancies.