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=head1 NAME
SQL::Amazon::UserGuide - User Guide for DBD/SQL::Amazon
=head1 SQL::Amazon SYNOPSIS
        #
        #       create the parser, passing in the current Amazon metadata
        #
        my $parser = SQL::Amazon::Parser->new(\%attrs);
        #
        #       parse a SQL statement, returning a SQL::Amazon::Statement
        #
        my $stmt = $parser->parse($sql_stmt)
                or die "Parse failed: " . $parser->errstr;
        #
        #       evaluate the parse tree, using an evaluation object
        #       for driver specific evaluation
        #       returns either a scalar rowcount for write operations,
        #               or a SQL::Amazon::Table object for SELECT
        #               or undef on error
        #
        my $results = $stmt->execute($params)
                or die "Evaluation failed: " . $stmt->errstr;
=head1 DESCRIPTION
SQL::Amazon provides the various components required by
B<DBD::Amazon> L<http://www.presicient.com/dbdamzn> to query the 
Amazon E-Commerce Service 4.0 I<aka ECS> using SQL. 
B<Be advised this is ALPHA release software>.
The suite includes the following components:
=over 4
=item B<SQL::Amazon::Parser>
provides SQL parsing and query plan generation. Implemented as a 
subclass of L<SQL::Parser>, part of the L<SQL::Statement> bundle.
=item B<SQL::Amazon::Statement>
provides SQL query plan execution. Implemented as a subclass of
L<SQL::Statement>.
=item B<SQL::Amazon::Functions>
provides SQL::Amazon-specific predicate functions, including 
B<MATCHES ANY, MATCHES ALL, MATCHES TEXT, POWER_SEARCH,
IN, and NOT IN>.
=item B<SQL::Amazon::ReqFactory>
provides a factory class for generating SQL::Amazon::Request::Request
objects based on the predicates in a query's WHERE clause.
=item B<SQL::Amazon::Spool>
provides a temporary storage object for intermediate results
extracted from the base table cache objects. Acts as a 
L<SQL::Eval::Table> object for L<SQL::Statement> processing.
=item B<SQL::Amazon::StorageEngine>
provides a global storage engine for managing data caching and
retrieval.
=item B<SQL::Amazon::Request::Request>
provides a base class for all ECS request objects, including numerous
default method implementations for building and sending requests,
and processing the responses into the base table cache objects.
=item B<SQL::Amazon::Request::ItemLookup>
a subclass of SQL::Amazon::Request::Request for the ItemLookup
request; also acts as a base class for the ItemSearch request.
=item B<SQL::Amazon::Request::ItemSearch>
a subclass of SQL::Amazon::Request::ItemLookup for the ItemSearch request
=item B<SQL::Amazon::Tables::Table>
provides a base class for table cache objects, including methods
for data type conversion, keyed lookup, and cache management.
=item B<SQL::Amazon::Tables::><B<tablename>>
provides table-specific implementations of the Table base class.
=back
=head2 Prerequisites
Perl 5.8.0
L<SQL::Statement> 1.10
L<Clone> 0.15
L<DBI> 1.47
L<XML::Simple> 2.09
L<LWP> I<from of libwww-perl 5.803>>
=head2 SQL Dialect
SQL::Amazon::Parser currently supports the same subset of standard 
SQL as supported by L<SQL::Statement>, using the 
L<SQL::Dialects::Amazon> dialect, with the following additional
predicate functions for keyword searches:
=over 4
=item B<MATCHES [ ANY | ALL | TEXT] (>E<lt>I<keyword-list>E<gt>B<)>
Results in generation of an ECS ItemSearch request, with either the
Keywords parameter (for MATCHES ALL), Power parameter (for MATCHES ANY),
or TextStream parameter (for MATCHES TEXT).
=item B<POWER_SEARCH(>E<lt>I<power-search-list>E<gt>B<)>
Results in generation of an ECS ItemSearch request, with either
Power parameters. Individual items in the I<power-search-list> are
wrapped in parentheses, and joined with 'and' operators.
=back
=head2 Special SQL Considerations
=head4 Mapping ECS Hierarchical Response Data to Relational Structures
The hierarchical structure of the data returned by the Amazon ECS requires
some manipulation by SQL::Amazon to conform to a flattened relational model. 
As a result, queries against the B<Books> table may cause multiple tables 
to be populated, e.g., 
B<Offers, CustomerReviews, EditorialReviews, ListManiaLists>, etc. 
This behavior is especially true when the B<ResponseGroup> parameter is set to 
B<Large> I<(the default)>, in which case all detail information about 
matching books is returned. 
As a result, some queries require the use of JOINs to properly
reflect the data model abstraction. E.g., When searching for only New books,
the B<Condition> ECS request parameter must be applied; however, B<Condition>
is a column in the B<Offers> table, B<not> the B<Books> table. Hence, a JOIN
between the B<Books> table and B<Offers> table on the B<ASIN> column is
required, along with a predicate test for B<Offers.Condition = 'New'>. 
The JOIN is required, even though the returned B<SELECT> column list may not 
contain any B<Offers> columns. Unfortunately, this requirement can have 
undesirable effects when the B<ResponseGroup> parameter has been set to 
'Small' or 'Medium' I<(via the DBD::Amazon amzn_resp_group attribute)>, as 
those response groups do not populate the B<Offers> table. In that situation, 
L<SQL::Statement>'s eventual application of INNER JOIN logic will result in no
JOIN'ed rows, and thus no results. In such cases, a LEFT JOIN may be required,
and, due to limitations of the current SQL::Statement JOIN logic,
the JOIN predicate must be placed within an B<ON> clause I<aka, an EXPLICIT JOIN>,
rather than within the B<WHERE> clause I<aka, an IMPLICIT JOIN>.
=head4 "Pseudo" Columns
Another special consideration is the support for some "pseudo" columns within
WHERE clauses that are not available for inclusion in the SELECT column list.
These pseudo-columns are used to generate some ECS request parameters which
do not have a direct element mappings in the ECS response data. The following
pseudo-columns are currently available for predicates, but not the SELECT
column list:
=over 4
=item Subject - subject area fo the book; used for power searches
=item Language - the language of the book; used for power searches
=back
=head4 Handling NULL Values for Request-mapped Columns
Under some circumstances I<usually when the ResponseGroup is set
to 'Medium' or 'Small'>, some fields are not returned in the
response, resulting in NULL values for those columns, which can create
an issue for filtering the results in the WHERE clause, as NULL
values in the usual comparison operators always evaluate to FALSE.
While adding an "...OR <column> IS NULL..." predicate may seem
a reasonable solution, the additional disjunction generated by
this addition can result in extraneous ECS requests, thereby greatly
increasing the processing time required. The best alternative in these
cases is to create either a B<COALESCE()> or B<DECODE()> alias for
the column in the SELECT returned column list.
=head4 Author Elements Combined Into a Single Authors Column
A single book item returned by ECS may include multiple Author elements.
Rather than flattening these elements into a separate table, multiple Author
elements are joined into a single string, using "; " as a separator, and
placed in the Authors column of the Books table.
=head4 Cached Versions of Tables
SQL::Amazon maintains a cache of ECS results, keyed by the request identifier
returned in each ECS response message. Normally, SQL::Amazon requires that
a SELECT statement includes a WHERE clause that provides sufficient 
predicates to form an ECS request, in order to retrieve data on which
the SELECT statement can further operate. In some instances, however,
it may be desirable to use the existing data returned for a prior 
SELECT statement, e.g., to do further drill-down on the results of
a prior SELECT during an ad-hoc query session. In support of this,
alternate named tables, prefixed with the word "Cached", can be used
to force the SELECT statement to operate only on the data currently in 
the cache for the specified table. E.g.,
        SELECT * FROM CachedBooks WHERE PublicationDate > '2002-01-01';
         
would return all the cached rows from Books with a 
PublicationDate > 2002-01-01, without issuing any further ECS requests.
B<Note> that using cached tables may result in duplicate rows in the
result set. Furthermore, due to the cache time limit requirements 
imposed by Amazon, rows may spoil out of the cache between consecutive 
references to a cached table.
=head2 Internals
Each statement handle created by B<DBD::Amazon> 
L<http://www.presicient.com/dbdamzn> creates its own 
SQL::Amazon::Statement object. All SQL::Amazon:Statement instances 
share a single L<SQL::Amazon::StorageEngine> object, which manages the 
service request and table abstraction processing, and maintains the 
cache of previously retrieved results (subject to the Amazon ECS 
license's caching rules), in order to limit the number of requests 
required to satisfy a request.
=head1 ACKNOWLEDGEMENTS
Many thanks to Jeff Zucker for his assistance/guidance/patience on using,
and accepting patches for, SQL::Statement.
=head1 FOR MORE INFO
L<http://www.presicient.com/dbdamzn>
=head1 AUTHOR AND COPYRIGHT
Copyright (C) 2005 by Presicient Corporation, USA
L<mailto:darnold@presicient.com>
L<http://www.presicient.com>
Permission is granted to use this software according to the terms of the
Artistic License, as specified in the Perl README file,
with the exception that commercial redistribution, either 
electronic or via physical media, as either a standalone package, 
or incorporated into a third party product, requires prior 
written approval of the author.
This software is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Presicient Corp. reserves the right to provide support for this software
to individual sites under a separate (possibly fee-based)
agreement.
=head1 SEE ALSO
L<SQL::Statement> for the supported SQL syntax, and how to subclass
L<SQL::Parser>.
B<DBD::Amazon> L<http://www.presicient.com/dbdamzn> for general usage 
instructions for the DBI interface to the Amazon ECS.
L<http://dbi.perl.org> for general information on DBI
   
L<http://www.amazon.com/gp> for information about the Amazon ECS API.
The DBI users mailing list L<mailto:dbi-users-subscribe@perl.org> for 
help on the use of DBI.
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