# *********************************************************************
#  Written by and copyright Carlo Strozzi <carlos@linux.it>.
#
#  column.txt: help text
#  Copyright (C) 2001 Carlo Strozzi <carlos@linux.it>
# 
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#  but WITHOUT ANY WARRANTY; without even the implied warranty of
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#  GNU General Public License for more details.
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#  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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# *********************************************************************

                      NoSQL operator: column

Selects columns by name (and order) and outputs a table with
these columns.

Usage: column [options] column [column ...]

Options:
    --input (-i) 'file'
      Read input from 'file' instead of STDIN.

    --output (-o) 'file'
      Write output to 'file' instead of STDOUT.

    --help (-h)
      Display this help text.

    --last (-l)
      If the input table contains duplicated column names
      pick the last occurrence of each. The default is to
      pick the first one. This is sometimes useful after
      the 'jointable' operator.

    --no-header (-N)
      Suppress table header from output.

    --null (-n) 'string'
      Set NULL input fields to 'string' on output.

    --like (-L) table
      Take the list of output columns from 'table'.

Notes:

Selects columns by name (and order) and outputs a table with
these columns. Can effectively select, order, delete, or duplicate
columns. If no columns are specified, then nothing is printed. If a
column name does not match any of the columns in table, a new column
with that name is inserted in that location. If no columns are
specified then all the input columns are printed to STDOUT, but any
duplicated ones are removed. This can be useful, possibly in
conjunction with '--last', to clean up the result of a 'jointable'
operation.

Comparing the AWK version of 'column' with the old one written
in C++ (which is still distributed in the contributed stuff)
it emerges that the former is actually faster than the latter
even on a large 20000-record table. Of course using mawk(1)
directly by referencing field positions is even faster.

$Id$
