How to: Insert Columns into Tables (Visual Database Tools)
Use Table Designer to add new columns to a table. When you open a table in Table Designer, you see all of its currently defined columns as well as a blank row at the bottom of the table definition grid. You can add columns either in the blank row or you can insert them between existing rows.
To insert columns into a table with Table Designer
In Object Explorer, right-click the table to which you want to add columns and choose Design.
Table Designer opens with the cursor placed in the first blank cell in the Column Name column. You can also right-click a row in the table and select Insert Column from the shortcut menu. A blank column row is inserted.
In the Column Name column, type the column name in the cell. The column name is a required value.
Press the TAB key to go to the Data Type cell and select a data type from the dropdown. This too is a required value, and will be assigned the default value if you don't choose one.
You can change the default value in the Options dialog box under Database Tools.
Continue to define any other column properties in the Column Properties tab.
The default values for your column properties are added when you create a new column, but you can change them in the Column Properties tab.