We wish to select a subset of the columns of a table. We wish to identify the columns that we wish to select by
specifying their name or their position. In particular:
Selection by Name where we identify the columns we wish to select by
specifying their names. This
is by far the most common column selection scenario.
Selection by Position where we identify the columns we wish to select
by specifying their position (as an integer). The positions start at 1 for the left most column.