Deleting Columns 
Prerequisites
You are logged on to the database as a database user.
You are the owner of the table or have the ALTER privilege for the table to which the column belongs.
More information: SQL Reference Manual, Privilege Specification (priv_spec)
The database is in the ONLINE operational state.
Procedure
Select the table in the explorer tree.
In the context menu of the table, choose Definition.
In the context menu of the column, choose Remove Column.
In the context menu of the editor window, choose Alter Table.
More Information
SQL Reference Manual, ALTER TABLE Statement (alter_table_definition)