How to add a calculated column to Access via SQL

问题

How do i add a calculated column to an Access table in SQL?

I know i can add a column with SQL like this:

ALTER TABLE Clients ADD COLUMN AccountDate TEXT(60)

Thanks, Vítor

回答1:

You cannot add a calculated column with SQL because calculated field requires an expression and that cannot be supplied through SQL. Technically, a calculated field is a base type - int, double, text etc. Above the base type is an expression that helps Access do the math/logic.

You could use VBA to create a calculated column

-- create a module and let's assume that
-- your table has Field1 integer and Field2 integer
-- we will add field3

Public Sub CreateField()

    Dim DB As DAO.Database
    Dim TableDef As DAO.TableDef
    Dim Fld As DAO.Field2

    Set DB = CurrentDb()
    Set TableDef = DB.TableDefs("Table1")

    Set Fld = TableDef.CreateField("field3", dbDouble)
    Fld.Expression = "[field1] * [field2]"
    TableDef.Fields.Append Fld

    MsgBox "Added"

End Sub

As Gordon and BJones mentioned, you could create a view or saved query with relevant calculation.

回答2:

I don't think MS Access supports computed columns. Instead, you can create a view:

create view v_clients as
    select c.*, (col1 + col2) as col3
    from clients;

回答3:

What do you want it to calculate? Have you tried something like:

ALTER TABLE Clients 
ADD COLUMN AccountDate AS (Column1 * Column2);

来源：https://stackoverflow.com/questions/31621363/how-to-add-a-calculated-column-to-access-via-sql