libs/DBD/CSV.pm

# -*- perl -*-
#
#   DBD::CSV - A DBI driver for CSV and similar structured files
#
#   This module is currently maintained by
#
#       Jeff Zucker
#       <jeff@vpservices.com>
#
#   The original author is Jochen Wiedmann.
#
#   Copyright (C) 1998 by Jochen Wiedmann
#
#   All rights reserved.
#
#   You may distribute this module under the terms of either the GNU
#   General Public License or the Artistic License, as specified in
#   the Perl README file.
#

require 5.004;
use strict;


require DynaLoader;
require DBD::File;
require IO::File;


package DBD::CSV;

use vars qw(@ISA $VERSION $drh $err $errstr $sqlstate);

@ISA = qw(DBD::File);

$VERSION = '0.2002';

$err = 0;               # holds error code   for DBI::err
$errstr = "";           # holds error string for DBI::errstr
$sqlstate = "";         # holds error state  for DBI::state
$drh = undef;           # holds driver handle once initialised


package DBD::CSV::dr; # ====== DRIVER ======

use Text::CSV_XS();

use vars qw(@ISA @CSV_TYPES);

@CSV_TYPES = (
    Text::CSV_XS::IV(), # SQL_TINYINT
    Text::CSV_XS::IV(), # SQL_BIGINT
    Text::CSV_XS::PV(), # SQL_LONGVARBINARY
    Text::CSV_XS::PV(), # SQL_VARBINARY
    Text::CSV_XS::PV(), # SQL_BINARY
    Text::CSV_XS::PV(), # SQL_LONGVARCHAR
    Text::CSV_XS::PV(), # SQL_ALL_TYPES
    Text::CSV_XS::PV(), # SQL_CHAR
    Text::CSV_XS::NV(), # SQL_NUMERIC
    Text::CSV_XS::NV(), # SQL_DECIMAL
    Text::CSV_XS::IV(), # SQL_INTEGER
    Text::CSV_XS::IV(), # SQL_SMALLINT
    Text::CSV_XS::NV(), # SQL_FLOAT
    Text::CSV_XS::NV(), # SQL_REAL
    Text::CSV_XS::NV(), # SQL_DOUBLE
);

@DBD::CSV::dr::ISA = qw(DBD::File::dr);

$DBD::CSV::dr::imp_data_size = 0;
$DBD::CSV::dr::data_sources_attr = undef;

sub connect ($$;$$$) {
    my($drh, $dbname, $user, $auth, $attr) = @_;

    my $this = $drh->DBD::File::dr::connect($dbname, $user, $auth, $attr);
    $this->{'csv_tables'} ||= {};

    $this;
}


package DBD::CSV::db; # ====== DATABASE ======

$DBD::CSV::db::imp_data_size = 0;

@DBD::CSV::db::ISA = qw(DBD::File::db);

sub csv_cache_sql_parser_object {
    my $dbh = shift;
    my $parser = {
            dialect    => 'CSV',
            RaiseError => $dbh->FETCH('RaiseError'),
            PrintError => $dbh->FETCH('PrintError'),
        };
    my $sql_flags  = $dbh->FETCH('csv_sql') || {};
    %$parser = (%$parser,%$sql_flags);
    $parser = SQL::Parser->new($parser->{dialect},$parser);
    $dbh->{csv_sql_parser_object} = $parser;
    return $parser;
}


package DBD::CSV::st; # ====== STATEMENT ======

$DBD::CSV::st::imp_data_size = 0;

@DBD::CSV::st::ISA = qw(DBD::File::st);


package DBD::CSV::Statement;

@DBD::CSV::Statement::ISA = qw(DBD::File::Statement);

sub open_table ($$$$$) {
    my($self, $data, $table, $createMode, $lockMode) = @_;

    # remember some attributes if scanning starts below
    $data->{Database}->{_cache}->{csv_tables}->{$table}->{'col_names'} = $data->{Database}->{csv_tables}->{$table}->{'col_names'}
      if $data->{Database}->{csv_tables}->{$table}->{'col_names'};

SCAN:
    if ($data->{f_stmt}->{command} eq 'SELECT' && $data->{Database}->{scan}) {
      # get rules from builtin rulebase if requested
      $data->{Database}->{'scan'} = _get_rules_autoscan() if $data->{Database}->{'scan'} == 1;
      delete $data->{Database}->{csv_tables}->{$table};
      # rules left on stack?
      if (my $rule = shift @{$data->{Database}->{'scan'}}) {
        $data->{Database}->{scan_count}++;
        # merge csv-options to table metadata:
        # foreach (keys %{$rule}) { $data->{Database}->{csv_tables}->{$table}->{$_} = $rule->{$_}; }
        # overwrite table metadata, (re-)set csv-options:
        $data->{Database}->{csv_tables}->{$table} = $rule;
      } else {
        die "Missing first row or scanrule not applied";
      }
    }

    my $dbh = $data->{Database};
    my $tables = $dbh->{csv_tables};
    if (!exists($tables->{$table})) {
        $tables->{$table} = {};
    }
    my $meta = $tables->{$table} || {};
    my $csv = $meta->{csv} || $dbh->{csv_csv};
    if (!$csv) {
        my $class = $meta->{class}  ||  $dbh->{'csv_class'}  ||
            'Text::CSV_XS';
        my %opts = ( 'binary' => 1 );
        $opts{'eol'} = $meta->{'eol'} || $dbh->{'csv_eol'} || "\015\012";
        $opts{'sep_char'} =
            exists($meta->{'sep_char'}) ? $meta->{'sep_char'} :
                exists($dbh->{'csv_sep_char'}) ? $dbh->{'csv_sep_char'} : ",";
        $opts{'quote_char'} =
            exists($meta->{'quote_char'}) ? $meta->{'quote_char'} :
                exists($dbh->{'csv_quote_char'}) ? $dbh->{'csv_quote_char'} :
                    '"';
        $opts{'escape_char'} =
            exists($meta->{'escape_char'}) ? $meta->{'escape_char'} :
                exists($dbh->{'csv_escape_char'}) ? $dbh->{'csv_escape_char'} :
                    '"';
        $opts{'always_quote'} =
            exists($meta->{'always_quote'}) ? $meta->{'always_quote'} :
                exists($dbh->{'csv_always_quote'}) ? $dbh->{'csv_always_quote'} : 0;
        $csv = $meta->{csv} = $class->new(\%opts);
    }
    my $file = $meta->{file}  ||  $table;
    my $tbl = $self->SUPER::open_table($data, $file, $createMode, $lockMode);
    if ($tbl) {
        $tbl->{'csv_csv'} = $csv;
        my $types = $meta->{types};
        if ($types) {
            # The 'types' array contains DBI types, but we need types
            # suitable for Text::CSV_XS.
            my $t = [];
            foreach (@{$types}) {
                if ($_) {
                    $_ = $DBD::CSV::CSV_TYPES[$_+6]  ||  Text::CSV_XS::PV();
                } else {
                    $_ = Text::CSV_XS::PV();
                }
                push(@$t, $_);
            }
            $tbl->{types} = $t;
        }
        if (!$createMode) {
            my($array, $skipRows);
            if (exists($meta->{skip_rows})) {
                $skipRows = $meta->{skip_rows};
            } else {
                $skipRows = exists($meta->{col_names}) ? 0 : 1;
            }
            if ($skipRows--) {
                if (!($array = $tbl->fetch_row($data))) {
          if ($data->{Database}->{'scan'}) {
            # if requested, try to help figuring out delimiters (just with SELECTs)
            $data->{Database}->{'scan_running'} = 1;
            goto SCAN;
          }
          my $die_msg = '';
          if ($data->{f_stmt}->{command} ne 'SELECT') {
            $die_msg = ' - Note: scan does only work with a successful SELECT prior using ' . $data->{f_stmt}->{command};
          }
          die "Missing first row" . $die_msg;
                }
                $tbl->{col_names} = $array;
                while ($skipRows--) {
                    $tbl->fetch_row($data);
                }
            }
            $tbl->{first_row_pos} = $tbl->{fh}->tell();
            $tbl->{size} = ($tbl->{fh}->stat)[7];

        # checkpoint: did we already slurp to the end of the file?
        # is this correct to be assumed as an error 
        # since it shouldn't occour while mungling with the first line(s)?
        if ( $tbl->{first_row_pos} == $tbl->{size} ) {
          $data->{Database}->{'scan_running'} = 1;
          $tbl->{fh}->setpos(0);
          goto SCAN;
        }

        # scan successful?
        if ($dbh->{'scan_running'}) {       
          #print "matched rule: ", $dbh->{scan_count}, "\n";
          # merge back cached attributes to local metadata
          foreach (keys %{$dbh->{_cache}->{csv_tables}->{$table}}) {
            $meta->{col_names} = $dbh->{_cache}->{csv_tables}->{$table}->{$_};
          }
          # patch csv options from table metadata into the scope of the Text::CSV_XS object
          if ($data->{f_stmt}->{command} eq 'INSERT' || $data->{f_stmt}->{command} eq 'UPDATE') {
            my $rule = $data->{Database}->{csv_tables}->{$table};
            foreach (keys %{$rule}) { $tbl->{csv_csv} = $rule->{$_}; }
          }
        }

            my $array;
            if (exists($meta->{col_names})) {
                $array = $tbl->{col_names} = $meta->{col_names};
            } elsif (!$tbl->{col_names}  ||  !@{$tbl->{col_names}}) {
                # No column names given; fetch first row and create default
                # names.
                my $a = $tbl->{cached_row} = $tbl->fetch_row($data);
                $array = $tbl->{'col_names'};
                for (my $i = 0;  $i < @$a;  $i++) {
                    push(@$array, "col$i");
                }
            }
            my($col, $i);
            my $columns = $tbl->{col_nums};
            foreach $col (@$array) {
                $columns->{$col} = $i++;
            }
        }
    }
    $tbl;
}


package DBD::CSV::Table;

@DBD::CSV::Table::ISA = qw(DBD::File::Table);

sub fetch_row ($$) {
    my($self, $data) = @_;
    my $fields;
    if (exists($self->{cached_row})) {
        $fields = delete($self->{cached_row});
    } else {
        $! = 0;
        my $csv = $self->{csv_csv};
        local $/ = $csv->{'eol'};
        $fields = $csv->getline($self->{'fh'});
        if (!$fields) {
            die "Error while reading file " . $self->{'file'} . ": $!" if $!;
            return undef;
        }
    }
    $self->{row} = (@$fields ? $fields : undef);
}

sub push_row ($$$) {
    my($self, $data, $fields) = @_;
    my($csv) = $self->{csv_csv};
    my($fh) = $self->{'fh'};
    #
    #  Remove undef from the right end of the fields, so that at least
    #  in these cases undef is returned from FetchRow
    #
    while (@$fields  &&  !defined($fields->[$#$fields])) {
        pop @$fields;
    }
    if (!$csv->print($fh, $fields)) {
        die "Error while writing file " . $self->{'file'} . ": $!";
    }
    1;
}
*push_names = \&push_row;


1;


__END__

=head1 NAME

DBD::CSV - DBI driver for CSV files

=head1 SYNOPSIS

    use DBI;
    $dbh = DBI->connect("DBI:CSV:f_dir=/home/joe/csvdb")
        or die "Cannot connect: " . $DBI::errstr;
    $sth = $dbh->prepare("CREATE TABLE a (id INTEGER, name CHAR(10))")
        or die "Cannot prepare: " . $dbh->errstr();
    $sth->execute() or die "Cannot execute: " . $sth->errstr();
    $sth->finish();
    $dbh->disconnect();


    # Read a CSV file with ";" as the separator, as exported by
    # MS Excel. Note we need to escape the ";", otherwise it
    # would be treated as an attribute separator.
    $dbh = DBI->connect(qq{DBI:CSV:csv_sep_char=\\;});
    $sth = $dbh->prepare("SELECT * FROM info");

    # Same example, this time reading "info.csv" as a table:
    $dbh = DBI->connect(qq{DBI:CSV:csv_sep_char=\\;});
    $dbh->{'csv_tables'}->{'info'} = { 'file' => 'info.csv'};
    $sth = $dbh->prepare("SELECT * FROM info");


=head1 WARNING

THIS IS ALPHA SOFTWARE. It is *only* 'Alpha' because the interface (API)
is not finalized. The Alpha status does not reflect code quality or
stability.


=head1 DESCRIPTION

The DBD::CSV module is yet another driver for the DBI (Database independent
interface for Perl). This one is based on the SQL "engine" SQL::Statement
and the abstract DBI driver DBD::File and implements access to
so-called CSV files (Comma separated values). Such files are mostly used for
exporting MS Access and MS Excel data.

See L<DBI(3)> for details on DBI, L<SQL::Statement(3)> for details on
SQL::Statement and L<DBD::File(3)> for details on the base class
DBD::File.


=head2 Prerequisites

The only system dependent feature that DBD::File uses, is the C<flock()>
function. Thus the module should run (in theory) on any system with
a working C<flock()>, in particular on all Unix machines and on Windows
NT. Under Windows 95 and MacOS the use of C<flock()> is disabled, thus
the module should still be usable,

Unlike other DBI drivers, you don't need an external SQL engine
or a running server. All you need are the following Perl modules,
available from any CPAN mirror, for example

  ftp://ftp.funet.fi/pub/languages/perl/CPAN/modules/by-module

=over 4

=item DBI

the DBI (Database independent interface for Perl), version 1.00 or
a later release

=item SQL::Statement

a simple SQL engine

=item Text::CSV_XS

this module is used for writing rows to or reading rows from CSV files.

=back


=head2 Installation

Installing this module (and the prerequisites from above) is quite simple.
You just fetch the archive, extract it with

    gzip -cd DBD-CSV-0.1000.tar.gz | tar xf -

(this is for Unix users, Windows users would prefer WinZip or something
similar) and then enter the following:

    cd DBD-CSV-0.1000
    perl Makefile.PL
    make
    make test

If any tests fail, let me know. Otherwise go on with

    make install

Note that you almost definitely need root or administrator permissions.
If you don't have them, read the ExtUtils::MakeMaker man page for details
on installing in your own directories. L<ExtUtils::MakeMaker>.

=head2

  The level of SQL support available depends on the version of
  SQL::Statement installed.  Any version will support *basic*
  CREATE, INSERT, DELETE, UPDATE, and SELECT statements.  Only
  versions of SQL::Statement 1.0 and above support additional
  features such as table joins, string functions, etc.  See the
  documentation of the latest version of SQL::Statement for details.

=head2 Creating a database handle

Creating a database handle usually implies connecting to a database server.
Thus this command reads

    use DBI;
    my $dbh = DBI->connect("DBI:CSV:f_dir=$dir");

The directory tells the driver where it should create or open tables
(a.k.a. files). It defaults to the current directory, thus the following
are equivalent:

    $dbh = DBI->connect("DBI:CSV:");
    $dbh = DBI->connect("DBI:CSV:f_dir=.");

(I was told, that VMS requires

    $dbh = DBI->connect("DBI:CSV:f_dir=");

for whatever reasons.)

You may set other attributes in the DSN string, separated by semicolons.


=head2 Creating and dropping tables

You can create and drop tables with commands like the following:

    $dbh->do("CREATE TABLE $table (id INTEGER, name CHAR(64))");
    $dbh->do("DROP TABLE $table");

Note that currently only the column names will be stored and no other data.
Thus all other information including column type (INTEGER or CHAR(x), for
example), column attributes (NOT NULL, PRIMARY KEY, ...) will silently be
discarded. This may change in a later release.

A drop just removes the file without any warning.

See L<DBI(3)> for more details.

Table names cannot be arbitrary, due to restrictions of the SQL syntax.
I recommend that table names are valid SQL identifiers: The first
character is alphabetic, followed by an arbitrary number of alphanumeric
characters. If you want to use other files, the file names must start
with '/', './' or '../' and they must not contain white space.


=head2 Inserting, fetching and modifying data

The following examples insert some data in a table and fetch it back:
First all data in the string:

    $dbh->do("INSERT INTO $table VALUES (1, "
             . $dbh->quote("foobar") . ")");

Note the use of the quote method for escaping the word 'foobar'. Any
string must be escaped, even if it doesn't contain binary data.

Next an example using parameters:

    $dbh->do("INSERT INTO $table VALUES (?, ?)", undef,
             2, "It's a string!");

Note that you don't need to use the quote method here, this is done
automatically for you. This version is particularly well designed for
loops. Whenever performance is an issue, I recommend using this method.

You might wonder about the C<undef>. Don't wonder, just take it as it
is. :-) It's an attribute argument that I have never ever used and
will be parsed to the prepare method as a second argument.


To retrieve data, you can use the following:

    my($query) = "SELECT * FROM $table WHERE id > 1 ORDER BY id";
    my($sth) = $dbh->prepare($query);
    $sth->execute();
    while (my $row = $sth->fetchrow_hashref) {
        print("Found result row: id = ", $row->{'id'},
              ", name = ", $row->{'name'});
    }
    $sth->finish();

Again, column binding works: The same example again.

    my($query) = "SELECT * FROM $table WHERE id > 1 ORDER BY id";
    my($sth) = $dbh->prepare($query);
    $sth->execute();
    my($id, $name);
    $sth->bind_columns(undef, \$id, \$name);
    while ($sth->fetch) {
        print("Found result row: id = $id, name = $name\n");
    }
    $sth->finish();

Of course you can even use input parameters. Here's the same example
for the third time:

    my($query) = "SELECT * FROM $table WHERE id = ?";
    my($sth) = $dbh->prepare($query);
    $sth->bind_columns(undef, \$id, \$name);
    for (my($i) = 1;  $i <= 2;   $i++) {
        $sth->execute($id);
        if ($sth->fetch) {
            print("Found result row: id = $id, name = $name\n");
        }
        $sth->finish();
    }

See L<DBI(3)> for details on these methods. See L<SQL::Statement(3)> for
details on the WHERE clause.

Data rows are modified with the UPDATE statement:

    $dbh->do("UPDATE $table SET id = 3 WHERE id = 1");

Likewise you use the DELETE statement for removing rows:

    $dbh->do("DELETE FROM $table WHERE id > 1");


=head2 Error handling

In the above examples we have never cared about return codes. Of course,
this cannot be recommended. Instead we should have written (for example):

    my($query) = "SELECT * FROM $table WHERE id = ?";
    my($sth) = $dbh->prepare($query)
        or die "prepare: " . $dbh->errstr();
    $sth->bind_columns(undef, \$id, \$name)
        or die "bind_columns: " . $dbh->errstr();
    for (my($i) = 1;  $i <= 2;   $i++) {
        $sth->execute($id)
            or die "execute: " . $dbh->errstr();
        if ($sth->fetch) {
            print("Found result row: id = $id, name = $name\n");
        }
    }
    $sth->finish($id)
        or die "finish: " . $dbh->errstr();

Obviously this is tedious. Fortunately we have DBI's I<RaiseError>
attribute:

    $dbh->{'RaiseError'} = 1;
    $@ = '';
    eval {
        my($query) = "SELECT * FROM $table WHERE id = ?";
        my($sth) = $dbh->prepare($query);
        $sth->bind_columns(undef, \$id, \$name);
        for (my($i) = 1;  $i <= 2;   $i++) {
            $sth->execute($id);
            if ($sth->fetch) {
                print("Found result row: id = $id, name = $name\n");
            }
        }
        $sth->finish($id);
    };
    if ($@) { die "SQL database error: $@"; }

This is not only shorter, it even works when using DBI methods within
subroutines.


=head2 Metadata

The following attributes are handled by DBI itself and not by DBD::File,
thus they all work as expected:

    Active
    ActiveKids
    CachedKids
    CompatMode             (Not used)
    InactiveDestroy
    Kids
    PrintError
    RaiseError
    Warn                   (Not used)

The following DBI attributes are handled by DBD::File:

=over 4

=item AutoCommit

Always on

=item ChopBlanks

Works

=item NUM_OF_FIELDS

Valid after C<$sth-E<gt>execute>

=item NUM_OF_PARAMS

Valid after C<$sth-E<gt>prepare>

=item NAME

Valid after C<$sth-E<gt>execute>; undef for Non-Select statements.

=item NULLABLE

Not really working. Always returns an array ref of one's, as DBD::CSV
doesn't verify input data. Valid after C<$sth-E<gt>execute>; undef for
non-Select statements.

=back

These attributes and methods are not supported:

    bind_param_inout
    CursorName
    LongReadLen
    LongTruncOk

In addition to the DBI attributes, you can use the following dbh
attributes:

=over 8

=item f_dir

This attribute is used for setting the directory where CSV files are
opened. Usually you set it in the dbh, it defaults to the current
directory ("."). However, it is overwritable in the statement handles.

=item csv_eol

=item csv_sep_char

=item csv_quote_char

=item csv_escape_char

=item csv_class

=item csv_csv

The attributes I<csv_eol>, I<csv_sep_char>, I<csv_quote_char> and
I<csv_escape_char> are corresponding to the respective attributes of the
Text::CSV_XS object. You want to set these attributes if you have unusual
CSV files like F</etc/passwd> or MS Excel generated CSV files with a semicolon
as separator. Defaults are "\015\012", ';', '"' and '"', respectively.

The attributes are used to create an instance of the class I<csv_class>,
by default Text::CSV_XS. Alternatively you may pass an instance as
I<csv_csv>, the latter takes precedence. Note that the I<binary>
attribute I<must> be set to a true value in that case.

Additionally you may overwrite these attributes on a per-table base in
the I<csv_tables> attribute.

=item csv_tables

This hash ref is used for storing table dependent metadata. For any
table it contains an element with the table name as key and another
hash ref with the following attributes:

=over 12

=item file

The tables file name; defaults to

    "$dbh->{f_dir}/$table"

=item eol

=item sep_char

=item quote_char

=item escape_char

=item class

=item csv

These correspond to the attributes I<csv_eol>, I<csv_sep_char>,
I<csv_quote_char>, I<csv_escape_char>, I<csv_class> and I<csv_csv>.
The difference is that they work on a per-table base.

=item col_names

=item skip_first_row

By default DBD::CSV assumes that column names are stored in the first
row of the CSV file. If this is not the case, you can supply an array
ref of table names with the I<col_names> attribute. In that case the
attribute I<skip_first_row> will be set to FALSE.

If you supply an empty array ref, the driver will read the first row
for you, count the number of columns and create column names like
C<col0>, C<col1>, ...

=back

=back

Example: Suggest you want to use F</etc/passwd> as a CSV file. :-)
There simplest way is:

    require DBI;
    my $dbh = DBI->connect("DBI:CSV:f_dir=/etc;csv_eol=\n;"
                           . "csv_sep_char=:;csv_quote_char=;"
                           . "csv_escape_char=");
    $dbh->{'csv_tables'}->{'passwd'} = {
        'col_names' => ["login", "password", "uid", "gid", "realname",
                        "directory", "shell"]
    };
    $sth = $dbh->prepare("SELECT * FROM passwd");

Another possibility where you leave all the defaults as they are and
overwrite them on a per table base:

    require DBI;
    my $dbh = DBI->connect("DBI:CSV:");
    $dbh->{'csv_tables'}->{'passwd'} = {
        'eol' => "\n",
        'sep_char' => ":",
        'quote_char' => undef,
        'escape_char' => undef,
        'file' => '/etc/passwd',
        'col_names' => ["login", "password", "uid", "gid", "realname",
                        "directory", "shell"]
    };
    $sth = $dbh->prepare("SELECT * FROM passwd");


=head2 Driver private methods

These methods are inherited from DBD::File:

=over 4

=item data_sources

The C<data_sources> method returns a list of subdirectories of the current
directory in the form "DBI:CSV:directory=$dirname". 

If you want to read the subdirectories of another directory, use

    my($drh) = DBI->install_driver("CSV");
    my(@list) = $drh->data_sources('f_dir' => '/usr/local/csv_data' );

=item list_tables

This method returns a list of file names inside $dbh->{'directory'}.
Example:

    my($dbh) = DBI->connect("DBI:CSV:directory=/usr/local/csv_data");
    my(@list) = $dbh->func('list_tables');

Note that the list includes all files contained in the directory, even
those that have non-valid table names, from the view of SQL. See
L<Creating and dropping tables> above.

=back


=head2 Data restrictions

When inserting and fetching data, you will sometimes be surprised: DBD::CSV
doesn't correctly handle data types, in particular NULLs. If you insert
integers, it might happen, that fetch returns a string. Of course, a string
containing the integer, so that's perhaps not a real problem. But the
following will never work:

    $dbh->do("INSERT INTO $table (id, name) VALUES (?, ?)",
             undef, "foo bar");
    $sth = $dbh->prepare("SELECT * FROM $table WHERE id IS NULL");
    $sth->execute();
    my($id, $name);
    $sth->bind_columns(undef, \$id, \$name);
    while ($sth->fetch) {
        printf("Found result row: id = %s, name = %s\n",
              defined($id) ? $id : "NULL",
              defined($name) ? $name : "NULL");
    }
    $sth->finish();

The row we have just inserted, will never be returned! The reason is
obvious, if you examine the CSV file: The corresponding row looks
like

    "","foo bar"

In other words, not a NULL is stored, but an empty string. CSV files
don't have a concept of NULL values. Surprisingly the above example
works, if you insert a NULL value for the name! Again, you find
the explanation by examining the CSV file:

    ""

In other words, DBD::CSV has "emulated" a NULL value by writing a row
with less columns. Of course this works only if the rightmost column
is NULL, the two rightmost columns are NULL, ..., but the leftmost
column will never be NULL!

See L<Creating and dropping tables> above for table name restrictions.


=head1 TODO

Extensions of DBD::CSV:

=over 4

=item CSV file scanner

Write a simple CSV file scanner that reads a CSV file and attempts
to guess sep_char, quote_char, escape_char and eol automatically.

=back

These are merely restrictions of the DBD::File or SQL::Statement
modules:

=over 4

=item Table name mapping

Currently it is not possible to use files with names like C<names.csv>.
Instead you have to use soft links or rename files. As an alternative
one might use, for example a dbh attribute 'table_map'. It might be a
hash ref, the keys being the table names and the values being the file
names.

=item Column name mapping

Currently the module assumes that column names are stored in the first
row. While this is fine in most cases, there should be a possibility
of setting column names and column number from the programmer: For
example MS Access doesn't export column names by default.

=back


=head1 KNOWN BUGS

=over 8

=item *

The module is using flock() internally. However, this function is not
available on platforms. Using flock() is disabled on MacOS and Windows
95: There's no locking at all (perhaps not so important on these
operating systems, as they are for single users anyways).

=back


=head1 AUTHOR AND COPYRIGHT

This module is currently maintained by

      Jeff Zucker
      <jeff@vpservices.com>

The original author is Jochen Wiedmann.

Copyright (C) 1998 by Jochen Wiedmann

All rights reserved.

You may distribute this module under the terms of either the GNU
General Public License or the Artistic License, as specified in
the Perl README file.

=head1 SEE ALSO

L<DBI(3)>, L<Text::CSV_XS(3)>, L<SQL::Statement(3)>

For help on the use of DBD::CSV, see the DBI users mailing list:

  http://www.isc.org/dbi-lists.html

For general information on DBI see

  http://www.symbolstone.org/technology/perl/DBI

=cut
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