计算字段，基于2个ID的SELECT

Question

I have a table, named FacilityDatabaseConnection, like-so, ID, FacilityID, DatabaseTypeID, ConnectionString

So given a FacilityID and DatabaseTypeID pair you'd get a ConnectionString. Both Facility and DatabaseType Tables have "Name" Fields. I would like to make a "Name" field in FacilityDatabaseConnection to do the following,

SELECT (dbo.Facility.Name+' - '+dbo.DatabaseType.Name) as Name
FROM dbo.FacilityDatabaseConnection
INNER JOIN dbo.Facility
 ON dbo.FacilityDatabaseConnection.FacilityID = dbo.Facility.ID
INNER JOIN dbo.DatabaseType
 ON dbo.FacilityDatabaseConnection.DatabaseTypeID = dbo.DatabaseType.ID

So that it returns "FacilityName - DatabaseType"

This works as a query but is it possible to make this a field?

---

I've tried,

ALTER TABLE dbo.FacilityDatabaseConnection
ADD Name AS (SELECT (dbo.Facility.Name+' - '+dbo.DatabaseType.Name) as Name
FROM dbo.FacilityDatabaseConnection
INNER JOIN dbo.Facility
 ON dbo.FacilityDatabaseConnection.FacilityID = dbo.Facility.ID
INNER JOIN dbo.DatabaseType
 ON dbo.FacilityDatabaseConnection.DatabaseTypeID = dbo.DatabaseType.ID) PERSISTED

Which gave me an error of "Subqueries are not allowed in this context. Only scalar expressions are allowed."

Is there a way to achieve this or is such a calculated field not possible?

Answer 1

Assuming this is SQL Server, computed columns cannot reference other tables, so what you are suggesting is not possible.

See also http://msdn.microsoft.com/en-us/library/ms191250%28v=sql.105%29.aspx

You should use a view/function/stored procedure instead.

Answer 2

There is a secret way to make computed column access another table.

That is to create a user-defined function that defines the field. The UDF can then access the other table.

The alter statement looks something like:

ALTER TABLE dbo.FacilityDatabaseConnection
    ADD Name AS udf_getFacilityName(FacilityId);

Answer 3

You can do the following

SELECT
    (SELECT count(*) FROM table1) as new_column_name,
    col1,
    col2
FROM
    table2