ADO - ADOX Column Object for Column Definition Management

The ADOX Column Object is used in ADOX (ActiveX Data Objects Extensions for Data Definition Language and Security) to work with the columns of database tables. While traditional ADO is mainly used to retrieve, insert, update, and delete data, ADOX provides additional functionality for managing the structure and schema of a database. The ADOX Column object represents an individual column within a database table and allows applications to examine or modify properties such as the column name, data type, size, and other column-related attributes.

Purpose of the ADOX Column Object

Every database table consists of one or more columns. For example, an Employees table might contain columns such as EmployeeID, EmployeeName, Department, and Salary. Each column has its own definition, including its name and data type. The ADOX Column object provides a programmatic way to access these definitions.

It is particularly useful when an application needs to dynamically inspect or create database structures rather than relying entirely on manually created tables. Developers can use the object to determine what columns exist in a table, retrieve their properties, add new columns, or remove existing columns.

Accessing Columns Through the Tables Collection

ADOX organizes database objects through collections. The Catalog object represents the database, while the Tables collection contains the tables in that database. Each Table object has a Columns collection containing its individual columns.

The general hierarchy can be understood as:

Catalog
   |
   +-- Tables
         |
         +-- Table
               |
               +-- Columns
                     |
                     +-- Column

For example, if a database contains an Employees table, the Columns collection of that table can be used to access individual column definitions.

A conceptual example is:

Dim cat As New ADOX.Catalog
Dim tbl As ADOX.Table
Dim col As ADOX.Column

cat.ActiveConnection = connection

Set tbl = cat.Tables("Employees")

For Each col In tbl.Columns
    Debug.Print col.Name
Next

This code connects an ADOX Catalog to a database, selects the Employees table, and then goes through its columns. The Name property of each Column object can be used to display the column name.

Important Properties of the Column Object

The ADOX Column object provides several properties that describe a column.

Name

The Name property identifies the column. For example:

EmployeeID
EmployeeName
Salary

The name is essential because applications use it to identify a particular column within a table.

Type

The Type property specifies the data type of the column. Depending on the database provider, this can represent types such as integer, string, date, decimal, or Boolean.

For example, an EmployeeID column might use an integer-compatible type, while EmployeeName might use a character or string type.

DefinedSize

DefinedSize represents the defined size of a column where the underlying data type supports a size. This is particularly relevant for character-based fields.

For example, a database may define:

EmployeeName VARCHAR(100)

Here, the defined size is 100 characters.

Precision

The Precision property can be used with numeric data types to indicate the precision associated with a column. It is useful when working with numeric database definitions where the total number of significant digits matters.

NumericScale

NumericScale represents the scale of a numeric column. For example, a decimal column could be defined conceptually as:

Salary DECIMAL(10,2)

The precision is 10 and the scale is 2, meaning two digits are reserved for the fractional portion.

Creating a New Column

One important use of the ADOX Column object is creating a column definition before adding it to a table.

For example:

Dim col As New ADOX.Column

col.Name = "Email"
col.Type = adVarWChar
col.DefinedSize = 150

tbl.Columns.Append col

Here, a new column called Email is created. Its data type is specified, its size is set, and it is then appended to the table's Columns collection.

The exact data type constants and capabilities available can depend on the ADO provider being used.

Adding a Column to an Existing Table

The Columns.Append method can be used to add a column to a table definition.

For example:

Dim col As New ADOX.Column

col.Name = "PhoneNumber"
col.Type = adVarWChar
col.DefinedSize = 20

tbl.Columns.Append col

After the operation succeeds, the table can contain the newly defined column.

This capability can be useful for applications that need to construct or modify database schemas dynamically.

Removing a Column

The Columns collection also supports removing an existing column from a table.

For example:

tbl.Columns.Delete "PhoneNumber"

This removes the specified column from the table definition, provided that the database provider permits the operation and there are no database constraints preventing it.

Column deletion should be performed carefully because it changes the database structure and may result in the loss of data stored in that column.

Column Object and Database Schema

A database schema describes how database objects are organized. Tables, columns, indexes, keys, and other objects form important parts of that structure.

The ADOX Column object focuses specifically on the definition of individual table fields. This makes it useful when an application needs to inspect the schema programmatically.

For example, an application could examine a table and determine:

Column Name       Data Type       Size
---------------------------------------
EmployeeID        Integer         -
EmployeeName      String          100
Department        String          50
Salary            Decimal         10,2

Instead of hard-coding this information, the application can retrieve it from the database through ADOX.

Column Attributes

Depending on the provider, a column can also expose additional attributes through its properties. These may describe characteristics such as whether a column permits null values, whether it has an automatic or incrementing behavior, or other provider-specific characteristics.

It is important to remember that ADOX is an abstraction over database providers. Therefore, not every database system supports every ADOX property or schema operation in exactly the same way.

Difference Between ADO and ADOX Column Management

ADO and ADOX have different primary purposes.

ADO is primarily concerned with working with data. It provides objects such as Connection, Command, and Recordset for executing commands and retrieving or modifying database information.

ADOX extends ADO with functionality for working with database definitions and schema-related objects.

For example:

ADO
 |
 +-- Connect to database
 +-- Execute commands
 +-- Retrieve records
 +-- Update records

Whereas:

ADOX
 |
 +-- Create tables
 +-- Manage columns
 +-- Work with indexes
 +-- Work with keys
 +-- Examine database structure

The ADOX Column object therefore belongs to the schema-management side of database programming.

Practical Example

Suppose an application creates an employee database dynamically. Initially, the Employees table contains:

EmployeeID
EmployeeName
Department

Later, the application requires an Email field. Instead of manually changing the database, the application can create an ADOX Column object:

Dim emailColumn As New ADOX.Column

emailColumn.Name = "Email"
emailColumn.Type = adVarWChar
emailColumn.DefinedSize = 150

tbl.Columns.Append emailColumn

The application can then inspect the table's Columns collection to verify that the new column exists.

Advantages

The ADOX Column object provides several benefits:

  1. Programmatic schema management: Database columns can be examined and modified through code.

  2. Dynamic database creation: Applications can construct database structures at runtime.

  3. Schema inspection: Developers can discover column names, types, and sizes without manually examining the database.

  4. Automation: Repetitive database-structure operations can be automated.

  5. Database administration support: Applications that manage database structures can use ADOX objects to work with schema information.

Limitations

ADOX functionality depends significantly on the database provider. Some providers may not implement all ADOX features or may expose certain properties differently. Consequently, code that works with one database system may require modifications when used with another.

Another consideration is that schema changes such as deleting or changing columns can affect existing data and database dependencies. Such operations should therefore be carefully tested before being performed on production databases.

Conclusion

The ADOX Column Object represents an individual column in a database table and provides programmatic access to its definition. Through the Columns collection, developers can inspect existing columns, create new column definitions, add columns to tables, and remove columns when supported by the database provider. Its primary role is database schema and structure management, making it different from the data-manipulation functionality traditionally associated with ADO.