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\renewcommand{\runningtitle}{SWI-Prolog ODBC Interface}

\begin{document}

\title{SWI-Prolog ODBC Interface}
\author{Jan Wielemaker \\
	SWI, \\
	University of Amsterdam \\
	The Netherlands \\
	E-mail: \email{jan@swi-prolog.org}}

\maketitle

\begin{abstract}
This document describes the SWI-Prolog interface to ODBC, the Microsoft
standard for \jargon{Open DataBase Connectivity}.  These days there are
ODBC managers from multiple vendors for many platforms as well as
drivers for most databases, making it an attractive target for a Prolog
database connection.

The database interface is envisioned to consist of two layers. The first
layer is an encapsulation of the core functionality of ODBC.  This layer
makes it possible to run SQL queries.  The second layer exploits the
relation between Prolog predicates and database tables, providing
---a somewhat limited--- natural Prolog view on the data. The current
interface only covers the first layer.
\end{abstract}

\pagebreak
\tableofcontents
\pagebreak

\section{Introduction}

The value of RDMS for Prolog is often over-estimated, as Prolog itself
can manage substantial amounts of data. Nevertheless a Prolog/RDMS
interface provides advantages if data is already provided in an RDMS,
data must be shared with other applications, there are strong
persistency requirements or there is too much data to fit in memory.

The popularity of ODBC makes it possible to design a single
foreign-language module that provides RDMS access for a wide variety
of databases on a wide variety of platforms. The SWI-Prolog RDMS
interface is closely modeled after the ODBC API.  This API is rather
low-level, but defaults and dynamic typing provided by Prolog give the
user quite simple access to RDMS, while the interface provides the best
possible performance given the RDMS independency constraint.

The Prolog community knows about various high-level connections between
RDMS and Prolog. We envision these layered on top of the ODBC connection
described here.


\section{The ODBC layer}

\subsection{Connection management}

The ODBC interface deals with a single ODBC environment with
multiple simultaneous connections.  The predicates in this section
deal with connection management.

\begin{description}
    \predicate{odbc_connect}{3}{+DSN, -Connection, +Options}
Create a new ODBC connection to data-source \arg{DSN} and return a
handle to this connection in \arg{Connection}.  The connection handle
is either an opaque structure or an atom of the \const{alias} option
is used.  In addition to the options below, options applicable to
odbc_set_connection/2 may be provided.

\begin{description}
    \termitem{user}{User}
Define the user-name for the connection.  This option must be present
if the database uses authorization.

    \termitem{password}{Password}
Provide a password for the connection.  Normally used in combination
with \term{user}{User}.

    \termitem{alias}{AliasName}
Use \arg{AliasName} as \arg{Connection} identifier, making the
connection available as a global resource.  A good choice is to
use the \arg{DSN} as alias.

    \termitem{open}{OpenMode}
If \arg{OpenMode} is \const{once} (default if an \const{alias} is
provided), a second call to open the same \arg{DSN} simply returns
the existing connection.  If \const{multiple} (default if there is
no alias name), a second connection to the same data-source is
opened.

    \termitem{mars}{+Bool}
If \const{true}, use Microsoft SQL server 2005 \jargon{mars} mode.
This is support for multiple concurrent statements on a connection without
requiring the dynamic cursor (which incurs an astounding 20-50x slowdown
of query execution!!). MARS is a new feature in SQL2k5 apparently, and
only works if you use the native driver. For the non-native driver,
specifying that it is enabled will have absolutely no effect.
\end{description}

The following example connects to the WordNet%
	\footnote{An SQL version of WordNet is available from
		  \url{http://wordnet2sql.infocity.cjb.net/}}
\cite{miller1990} database, using the connection alias \const{wordnet}
and opening the connection only once:

\begin{code}
open_wordnet :-
	odbc_connect('WordNet', _,
		     [ user(jan),
		       password(xxx),
		       alias(wordnet),
		       open(once)
		     ]).
\end{code}

\predicate{odbc_driver_connect}{3}{+DriverString, -Connection, +Options}
Connects to a database using SQLDriverConnect(). This API allows for
driver-specific additional options. DriverString is passed without
checking. Options should \emph{not} include \const{user} and
\const{password}.

Whenever possible, applications should use odbc_connect/3. If you need
this predicate, please check the documentation for SQLDriverConnect()
and the documentation of your driver.%
\bug{Facilities to deal with prompted completion of the
     driver options are not yet implemented.}

\predicate{odbc_disconnect}{1}{+Connection}
Close the given \arg{Connection}.  This destroys the connection alias
or, if there is no alias, makes further use of the \arg{Connection}
handle illegal.

\predicate{odbc_current_connection}{2}{?Connection, ?DSN}
Enumerate the existing ODBC connections.

\predicate{odbc_set_connection}{2}{+Connection, +Option}
Set options on an existing connection.  All options defined here may
also be specified with odbc_connect/2 in the option-list.
Defined options are:

\begin{description}
    \termitem{access_mode}{Mode}
If \const{read}, tell the driver we only access the database in read
mode.  If \const{update} (default), tell the driver we may execute
update commands.

    \termitem{auto_commit}{bool}
If \const{true} (default), each update statement is committed
immediately.  If \const{false}, an update statement starts a transaction
that can be committed or rolled-back.  See \secref{sqltrans} for details
on transaction management.

    \termitem{cursor_type}{CursorType}
I haven't found a good description of what this does, but setting it
to \const{dynamic} makes it possible to have multiple active statements
on the same connection with Microsoft SQL server.  Other values
are \const{static}, \const{forwards_only} and \const{keyset_driven}.

    \termitem{silent}{Bool}
If \const{true} (default \const{false}), statements returning
\const{SQL_SUCCESS_WITH_INFO} succeed without printing the info.
See also \secref{successwithinfo}.

    \termitem{null}{NullSpecifier}
Defines how the SQL constant NULL is represented. Without specification,
the default is the atom \verb|$null$|. \arg{NullSpecifier} is an
arbitrary Prolog term, though the implementation is optimised for
using an unbound variable, atom and functor with one unbound variable.
The representation \term{null}{_} is a commonly used alternative.

The specified default holds for all statements executed on this
connection. Changing the connection default does not affect already
prepared or running statements.  The null-value can also be specified
at the statement level.  See the option list of odbc_query/4.

    \termitem{wide_column_threshold}{+Length}
If the width of a column exceeds \arg{Length}, use the API SQLGetData()
to get the value incrementally rather than using a (large) buffer
allocated with the statement.  The default is to use this alternate
interface for columns larger than 1024 bytes.  There are two cases
for using this option.  In time critical applications with wide columns
it may provide better performance at the cost of a higher memory usage
and to work around bugs in SQLGetData().  The latter applies to Microsoft
SQL Server fetching the definition of a view.
\end{description}

    \predicate{odbc_get_connection}{2}{+Connection, ?Property}
Query for properties of the connection.  \arg{Property} is a term
of the format \term{\arg{Name}}{\arg{Value}}.  If \arg{Property}
is unbound all defined properties are enumerated on backtracking.
Currently the following properties are defined.

\begin{description}
    \termitem{database_name}{Atom}
Name of the database associated to the connection.
    \termitem{dbms_name}{Name}
Name of the database engine.  This constant can be used to identify
the engine.
    \termitem{dbms_version}{Atom}
Version identifier from the database engine.
    \termitem{driver_name}{Name}
ODBC Dynamic Link Library providing the interface between ODBC and
the database.
    \termitem{driver_odbc_version}{Atom}
ODBC version supported by the driver.
    \termitem{driver_version}{Atom}
The drivers version identifier.
    \termitem{active_statements}{Integer}
Maximum number of statements that can be active at the same time on
this connection.  Returns 0 (zero) if this is unlimited.%
	\footnote{Microsoft SQL server can have multiple active
		  statements after setting the option
		  \const{cursor_type} to \const{dynamic}.  See
		  odbc_set_connection/2.}
\end{description}
    \predicate{odbc_data_source}{2}{?DSN, ?Description}
Query the defined data sources.  It is not required to have any open
connections before calling this predicate. \arg{DSN} is the name
of the data source as required by odbc_connect/3. \arg{Description} is
the name of the driver.  The driver name may be used to tailor the
SQL statements used on the database.  Unfortunately this name depends
on the local installing details and is therefore not universally
useful.
\end{description}

\subsection{Running SQL queries}

ODBC distinguishes between direct execution of literal SQL strings and
parameterized execution of SQL strings. The first is a simple practical
solution for infrequent calls (such as creating a table), while
parameterized execution allows the driver and database to precompile the
query and store the optimized code, making it suitable for time-critical
operations.  In addition, it allows for passing parameters without going
through SQL-syntax and thus avoiding the need for quoting.


\subsubsection{One-time invocation}

\begin{description}
    \predicate{odbc_query}{3}{+Connection, +SQL, -RowOrAffected}
Same as odbc_query/4 using \const{[]} for \arg{Options}.

    \predicate{odbc_query}{4}{+Connection, +SQL, -RowOrAffected, +Options}
Fire an SQL query on the database represented by \arg{Connection}.
\arg{SQL} is any valid SQL statement. SQL statements can be specified as
a plain atom, string or a term of the format
\mbox{\arg{Format}-\arg{Arguments}}, which is converted using format/2.

If the statement is a \const{SELECT} statement the result-set is
returned in \arg{RowOrAffected}. By default rows are returned one-by-one
on backtracking as terms of the functor \functor{row}{\arg{Arity}},
where \arg{Arity} denotes the number of columns in the result-set. The
library pre-fetches the next value to be able to close the statement and
return deterministic success when returning the last row of the
result-set. Using the option \functor{findall}{2} (see below) the
result-set is returned as a list of user-specified terms. For other
statements this argument returns \term{affected}{Rows}, where \arg{Rows}
represents the number of rows affected by the statement. If you are not
interested in the number of affected rows odbc_query/2 provides a simple
interface for sending SQL-statements.

Below is a small example using the connection created from
odbc_connect/3. Please note that the SQL-statement does not end in the
`\chr{;}' character.

\begin{code}
lemma(Lemma) :-
	odbc_query(wordnet,
		   'SELECT (lemma) FROM word',
		   row(Lemma).
\end{code}

The following example adds a name to a table with parent-relations,
returning the number of rows affected by the statement.

\begin{code}
insert_child(Child, Mother, Father, Affected) :-
	odbc_query(parents,
		   'INSERT INTO parents (name,mother,father) \
		      VALUES ("mary", "christine", "bob")',
		   affected(Affected)).
\end{code}

\arg{Options} defines the following options.

\begin{description}
    \termitem{types}{ListOfTypes}
Determine the Prolog type used to report the column-values.  When
omitted, default conversion as described in \secref{sqltypes} is
implied.  A column may specify \const{default} to use default
conversion for that column.  The length of the type-list must match
the number of columns in the result-set.

For example, in the table \exam{word} the first column is defined
with the SQL type \exam{DECIMAL(6)}.  Using this SQL-type, ``001''
is distinct from ``1'', but using Prolog integers is a valid
representation for Wordnet \exam{wordno} identifiers.  The following
query extracts rows using Prolog integers:

\begin{code}
?- odbc_query(wordnet,
	      'select * from word', X,
	      [ types([integer,default])
	      ]).

X = row(1, entity) ;
X = row(2, thing) ;
...
\end{code}

See also \secref{sqltypes} for notes on type-conversion.

    \termitem{null}{NullSpecifier}
Specify SQL NULL representation.  See odbc_set_connection/2 for details.

    \termitem{source}{Bool}
If \const{true} (default \const{false}), include the source-column with
each result-value. With this option, each result in the
\functor{row}{\arg{N}}-term is of the format below. \arg{TableName} or
\arg{ColumnName} may be the empty atom if the information is not
available.%
	\footnote{This is one possible interface to this information.
		  In many cases it is more efficient and convenient to
		  provide this information separately as it is the same
		  for each result-row.}

\begin{quote}
\term{column}{TableName, ColumnName, Value}
\end{quote}

    \termitem{findall}{Template, row(Column, \ldots)}
Instead of returning rows on backtracking this option makes odbc_query/3
return all rows in a list and close the statement.  The option is named
after the Prolog findall/3 predicate, as the it makes odbc_query/3
behave as the commonly used findall/3 construct below.

\begin{code}
lemmas(Lemmas) :-
	findall(Lemma,
		odbc_query(wordnet,
			   'select (lemma) from word',
			   row(Lemma)),
	        Lemmas).
\end{code}

Using the \functor{findall}{2} option the above can be implemented as
below.  The number of argument of the \const{row} term must match the
number of columns in the result-set.

\begin{code}
lemmas(Lemmas) :-
	odbc_query(wordnet,
		   'select (lemma) from word',
		   Lemmas,
		   [ findall(Lemma, row(Lemma))
		   ]).
\end{code}

\begin{quote}\it
The current implementation is incomplete. It does not allow arguments of
\term{row}{\ldots} to be instantiated. Plain instantiation can always
be avoided using a proper SELECT statement. Potentially useful however
would be the translation of compound terms, especially to translate
date/time/timestamp structures to a format for use by the application.
\end{quote}

    \termitem{wide_column_threshold}{+Length}
Specify threshold column width for using SQLGetData().
See odbc_set_connection/2 for details.
\end{description}

    \predicate{odbc_query}{2}{+Connection, +SQL}
As odbc_query/3, but used for SQL-statements that should not return
result-rows (i.e.\ all statements except for \const{SELECT}).  The
predicate prints a diagnostic message if the query returns a result.
\end{description}


\subsubsection{Parameterised queries}

ODBC provides for `parameterized queries'.  These are SQL queries with
a \chr{?}-sign at places where parameters appear.  The ODBC interface
and database driver may use this to precompile the SQL-statement, giving
better performance on repeated queries.  This is exactly what we want if
we associate Prolog predicates to database tables.  This interface is
defined by the following predicates:

\begin{description}
\predicate{odbc_prepare}{4}{+Connection, +SQL, +Parameters, -Statement}
As odbc_prepare/5 using \const{[]} for \arg{Options}.

\predicate{odbc_prepare}{5}{+Connection, +SQL, +Parameters,
			    -Statement, +Options}
Create a statement from the given \arg{SQL} (which may be a format
specification as described with odbc_query/3) statement that normally
has one or more parameter-indicators (\chr{?}) and unify \arg{Statement}
with a handle to the created statement. \arg{Parameters} is a list of
descriptions, one for each parameter. Each parameter description is one
of the following:

\begin{description}
    \termitem{default}{}
Uses the ODBC function SQLDescribeParam() to obtain information about
the parameter and apply default rules.  See \secref{sqltypes} for
details.  If the interface fails to return a type or the type is
unknown to the ODBC interface a message is printed and the interface
handles the type as text, which implies the user must supply an atom.
The message can be suppressed using the \term{silent}{true} option
of odbc_set_connection/2.  An alternative mapping can be selected
using the $>$ option of this predicate described below.

    \termitem{\arg{SqlType}}{Specifier, ...}
Declare the parameter to be of type \arg{SqlType} with the given
specifiers. Specifiers are required for \type{char}, \type{varchar},
etc.\ to specify the field-width. When calling odbc_execute/[2-3],
the user must supply the parameter values in the default Prolog type for
this SQL type. See \secref{sqltypes} for details.

    \definition{\arg{PrologType} $>$ \arg{SqlType}}
As above, but supply values of the given \arg{PrologType}, using the
type-transformation defined by the database driver.  For example,
if the parameter is specified as

\begin{code}
atom > date
\end{code}

The use must supply an atom of the format \exam{YYYY-MM-DD} rather than
a term \term{date}{Year,Month,Day}. This construct enhances flexibility
and allows for passing values that have no proper representation in
Prolog.
\end{description}

\arg{Options} defines a list of options for executing the statement.  See
odbc_query/4 for details. In addition, the following option is provided:

    \begin{description}
        \termitem{fetch}{FetchType}
Determine the \arg{FetchType}, which is one of \const{auto} (default) to
extract the result-set on backtracking or \const{fetch} to prepare the
result-set to be fetched using odbc_fetch/3.
    \end{description}

    \predicate{odbc_execute}{3}{+Statement, +ParameterValues, -RowOrAffected}
Execute a statement prepared with odbc_prepare/4 with the given
\arg{ParameterValues} and return the rows or number of affected rows
as odbc_query/4. This predicate may return type_error exceptions if the
provided parameter values cannot be converted to the declared types.

ODBC doesn't appear to allow for multiple cursors on the same
result-set.%
	\footnote{Is this right?}
This would imply there can only be one active odbc_execute/3
(i.e.\ with a choice-point) on a prepared statement.  Suppose we
have a table \exam{age (name char(25), age integer)} bound to the
predicate \predref{age}{2} we cannot write the code below without
special precautions.  The ODBC interface therefore creates a clone
of a statement if it discovers the statement is being executed,
which is discarded after the statement is finished.%
	\footnote{The code is prepared to maintain a cache of
		  statements.  Practice should tell us whether
		  it is worthwhile activating this.}

\begin{code}
same_age(X, Y) :-
	age(X, AgeX),
	age(Y, AgeY),
	AgeX = AgeY.
\end{code}

    \predicate{odbc_execute}{2}{+Statement, +ParameterValues}
Like odbc_query/2, this predicate is meant to execute simple SQL
statements without interest in the result.

    \predicate{odbc_free_statement}{1}{+Statement}
Destroy a statement prepared with odbc_prepare/4. If the statement is
currently executing (i.e. odbc_execute/3 left a choice-point), the
destruction is delayed until the execution terminates.
\end{description}


\subsubsection{Fetching rows explicitely}	\label{sec:sqlfetch}

Normally SQL queries return a result-set that is enumerated on
backtracking.  Using this approach a result-set is similar to a
predicate holding facts.  There are some cases where fetching the
rows one-by-one, much like read/1 reads terms from a file is more
appropriate and there are cases where only part of the result-set
is to be fetched.  These cases can be dealt with using odbc_fetch/3,
which provides an interface to SQLFetchScroll().

As a general rule of thumb, stay away from these functions if you do
not really need them.  Experiment before deciding on the strategy and
often you'll discover the simply backtracking approach is much easier
to deal with and about as fast.

\begin{description}
    \predicate{odbc_fetch}{3}{+Statement, -Row, +Option}
Fetch a row from the result-set of \arg{Statement}.  \arg{Statement}
must be created with odbc_prepare/5 using the option \term{fetch}{fetch}
and be executed using odbc_execute/2.  \arg{Row} is unified to the
fetched row or the atom \const{end_of_file}%
	\footnote{This atom was selected to emphasise the similarity
		  to read.}
after the end of the data is reached.  Calling odbc_fetch/2 after all
data is retrieved causes a permission-error exception. \arg{Option} is
one of:

    \begin{description}
	\termitem{next}{}
Fetch the next row.
	\termitem{prior}{}
Fetch the result-set going backwards.
	\termitem{first}{}
Fetch the first row.
	\termitem{last}{}
Fetch the last row.
	\termitem{absolute}{Offset}
Fetch absolute numbered row.  Rows count from one.
	\termitem{relative}{Offset}
Fetch relative to the current row.  \term{relative}{1} is the same
as \term{next}{}, except that the first row extracted is row 2.
	\termitem{bookmark}{Offset}
Reserved.  Bookmarks are not yet supported in this interface.
    \end{description}

In many cases, depending on the driver and RDBMS, the cursor-type
must be changed using odbc_set_connection/2 for anything different
from \term{next}{} to work.

Here is example code each time skipping a row from a table `test'
holding a single column of integers that represent the row-number.
This test was executed using unixODBC and MySQL on SuSE Linux.

\begin{code}
fetch(Options) :-
	odbc_set_connection(test, cursor_type(static)),
	odbc_prepare(test,
		     'select (testval) from test',
		     [],
		     Statement,
		     [ fetch(fetch)
		     ]),
	odbc_execute(Statement, []),
	fetch(Statement, Options).

fetch(Statement, Options) :-
	odbc_fetch(Statement, Row, Options),
	(   Row == end_of_file
	->  true
	;   writeln(Row),
	    fetch(Statement, Options)
	).
\end{code}

    \predicate{odbc_close_statement}{+Statement}
Closes the given statement (without freeing it).  This must be used
if not the whole result-set is retrieved using odbc_fetch/3.
\end{description}


\subsection{Transaction management}		\label{sec:sqltrans}

ODBC can run in two modi. By default, all update actions are immediately
committed on the server. Using odbc_set_connection/2 this behaviour can
be switched off, after which each SQL statement that can be inside a
transaction implicitly starts a new transaction. This transaction can be
ended using odbc_end_transaction/2.

\begin{description}
    \predicate{odbc_end_transaction}{2}{+Connection, +Action}
End the currently open transaction if there is one.  Using \arg{Action}
\const{commit} pending updates are made permanent, using
\const{rollback} they are discarded.
\end{description}

The ODBC documentation has many comments on transaction management and
its interaction with database cursors.


\subsection{Accessing the database dictionary}

With this interface we do not envision the use of Prolog as a database
manager.  Nevertheless, elementary access to the structure of a database
is required, for example to validate a database satisfies the
assumptions made by the application.

\begin{description}
    \predicate{odbc_current_table}{2}{+Connection, -Table}
Return on backtracking the names of all tables in the database
identified by the connection.

    \predicate{odbc_current_table}{3}{+Connection, ?Table, ?Facet}
Enumerate properties of the tables.  Defines facets are:

\begin{description}
    \termitem{qualifier}{Qualifier}
    \termitem{owner}{Owner}
    \termitem{comment}{Comment}
These facets are defined by SQLTables()
    \termitem{arity}{Arity}
This facet returns the number of columns in a table.
\end{description}

    \predicate{odbc_table_column}{3}{+Connection, ?Table, ?Column}
On backtracking, enumerate all columns in all tables.

    \predicate{odbc_table_column}{3}{+Connection, ?Table, ?Column, ?Facet}
Provides access to the properties of the table as defined by the ODBC
call SQLColumns().  Defined facets are:

\begin{description}
    \termitem{table_qualifier}{Qualifier}
    \termitem{table_owner}{Owner}
    \termitem{table_name}{Table}
See odbc_current_table/3.
    \termitem{data_type}{DataType}
    \termitem{type_name}{TypeName}
    \termitem{precision}{Precision}
    \termitem{length}{Length}
    \termitem{scale}{Scale}
    \termitem{radix}{Radix}
    \termitem{nullable}{Nullable}
    \termitem{remarks}{Remarks}
These facets are defined by SQLColumns()
    \termitem{type}{Type}
More prolog-friendly representation of the type properties.  See
\secref{sqltypes}.
\end{description}

    \predicate{odbc_type}{3}{+Connection, ?TypeSpec, ?Facet}
Query the types supported by the data source.  \arg{TypeSpec} is either
an integer type-id, the name of an ODBC SQL type or the constant
\const{all_types} to enumerate all known types.  This predicate calls
SQLGetTypeInfo() and its facet names are derived from the specification
of this ODBC function:

\begin{description}
    \termitem{name}{Name}
Name used by the data-source.  Use this in CREATE statements
    \termitem{data_type}{DataType}
Numeric identifier of the type
    \termitem{precision}{Precision}
When available, maximum precision of the type.
    \termitem{literal_prefix}{Prefix}
When available, prefix for literal representation.
    \termitem{literal_suffix}{Suffix}
When available, suffix for literal representation.
    \termitem{create_params}{CreateParams}
When available, arguments needed to create the type.
    \termitem{nullable}{Bool}
Whether the type can be \const{NULL}.  May be \const{unknown}
    \termitem{case_sensitive}{Bool}
Whether values for this type are case-sensitive.
    \termitem{searchable}{Searchable}
Whether the type can be searched. Values are \const{false},
\const{true}, \const{like_only} or \const{all_except_like}.
    \termitem{unsigned}{Bool}
When available, whether the value is signed. Please note that SWI-Prolog
does not provide unsigned integral values.
    \termitem{money}{Bool}
Whether the type represents money.
    \termitem{auto_increment}{Bool}
When available, whether the type can be auto-incremented.
    \termitem{local_name}{LocalName}
Name of the type in local language.
    \termitem{minimum_scale}{MinScale}
Minimum scale of the type.
    \termitem{maximum_scale}{MaxScale}
Maximum scale of the type.
\end{description}
\end{description}


\subsection{Getting more information}		\label{sec:odbcinfo}

\begin{description}
    \predicate{odbc_statistics}{1}{?Key}
Get statistical data on the ODBC interface.  Currently defined keys are:

\begin{description}
    \termitem{statements}{Created, Freed}
Number of SQL statements that have been \arg{Created} and \arg{Freed}
over all connections. Statements executed with odbc_query/[2-3]
increment \arg{Created} as the query is created and \arg{Freed} if
the query is terminated due to deterministic success, failure, cut
or exception.  Statements created with odbc_prepare/[4-5] are freed
by odbc_free_statement/1 or due to a fatal error with the statement.
\end{description}

    \predicate{odbc_debug}{1}{+Level}
Set the verbosity-level to \arg{Level}.  Default is 0.  Higher levels
make the system print debugging messages.
\end{description}


\subsection{Representing SQL data in Prolog}		\label{sec:sqltypes}

Databases have a poorly standardized but rich set of datatypes.  Some
have natural Prolog counterparts, some not.  A complete mapping requires
us to define Prolog data-types for SQL types that have no standardized
Prolog counterpart (such as timestamp), the definition of a default
mapping and the possibility to define an alternative mapping for a
specific column. For example, many variations of the SQL \const{DECIMAL}
type cannot be mapped to a Prolog integer.  Nevertheless, mapping to
an integer may be the proper choice for a specific application.

The Prolog/ODBC interface defines the following Prolog result types with
the indicated default transformation. Different result-types can be
requested using the \term{types}{TypeList} option for the
odbc_query/4 and odbc_prepare/5 interfaces.

\begin{description}
    \termitem{atom}{}
Used as default for the SQL types \const{char}, \const{varchar},
\const{longvarchar}, \const{binary}, \const{varbinary},
\const{longvarbinary}, \const{decimal} and \const{numeric}. Can be used
for all types.

    \termitem{string}{}
SWI-Prolog extended type string. Use the type for special cases where
garbage atoms must be avoided. Can be used for all types.

    \termitem{codes}{}
List of character codes.  Use this type if the argument must be analysed
or compatibility with Prolog systems that cannot handle infinite-length
atoms is desired.  Can be used for all types.

    \termitem{integer}{}
Used as default for the SQL types \const{bit}, \const{tinyint},
\const{smallint} and \const{integer}.  Please note that SWI-Prolog
integers are signed 32-bit values, where SQL allows for unsigned
values as well. Can be used for the integral, and \const{decimal} types
as well as the types \const{date} and \const{timestamp}, which are
represented as POSIX time-stamps (seconds after Jan 1, 1970).

    \termitem{double}{}
Used as default for the SQL types \const{real}, \const{float} and
\const{double}.  Can be used for the integral, float and \const{decimal}
types as well as the types \const{date} and \const{timestamp}, which
are represented as POSIX time-stamps (seconds after Jan 1, 1970).
Representing time this way is compatible to SWI-Prologs time-stamp
handling.

    \termitem{date}{}
A Prolog term of the form \term{date}{Year,Month,Day} used as default
for the SQL type \const{date}.

    \termitem{time}{}
A Prolog term of the form \term{time}{Hour,Minute,Second} used as
default for the SQL type \const{time}.

    \termitem{timestamp}{}
A Prolog term of the form
\term{timestamp}{Year,Month,Day,Hour,Minute,Second,Fraction} used as
default for the SQL type \const{timestamp}.
\end{description}


\subsection{Errors and warnings}

ODBC operations return success, error or `success with info'. This
section explains how results from the ODBC layer are reported to Prolog.

\subsubsection{ODBC messages: `Success with info'}
\label{sec:successwithinfo}

If an ODBC operation returns `with info', the info is extracted from the
interface and handled to the Prolog message dispatcher print_message/2.
The level of the message is \const{informational} and the term is of the
form:

\begin{description}
    \termitem{odbc}{State, Native, Message}
Here, \arg{State} is the SQL-state as defined in the ODBC API,
\arg{Native} is the (integer) error code of the underlying data source
and \arg{Message} is a human readable explanation of the message.
\end{description}

\subsubsection{ODBC errors}				\label{sec:sqlerror}

If an ODBC operation signals an error, it throws the exception
\term{error}{\term{odbc}{State, Native, Message}, _}. The arguments
of the \functor{odbc}{3} term are explained in \secref{successwithinfo}.

In addition, the Prolog layer performs the normal tests for proper
arguments and state, signaling the conventional instantiation, type,
domain and resource exceptions.


\subsection{ODBC implementations}

There is a wealth on ODBC implementations that are completely or almost
compatible to this interface. In addition, a number of databases are
delivered with an ODBC compatible interface. This implies you get the
portability benefits of ODBC without paying the configuration and
performance price. Currently this interface is, according to the
\url[PHP]{http://www.php.net} documentation on this subject, provided by
Adabas D, IBM DB2, Solid, and Sybase SQL Anywhere.


\subsubsection{Using unixODBC}

The SWI-Prolog ODBC interface was developed using
\url[unixODBC]{http://www.unixodbc.org} and \url[MySQL]{http://www.mysql.com} on
\url[SuSE Linux]{http://www.suse.com}.


\subsubsection{Using Microsoft ODBC}

On MS-Windows, the ODBC interface is a standard package, linked against
\file{odbc32.lib}.


\subsection{Remaining issues}

The following issues are identified and waiting for concrete problems
and suggestions.

\begin{description}
    \item[Transaction management]
This certainly requires a high-level interface.  Possibly in combination
with call_cleanup/3, providing automatic rollback on failure or
exception and commit on success.

    \item[High-level interface]
Attaching tables to predicates, partial \emph{DataLog} implementation,
etc.
\end{description}


\section{Installation}

\subsection{Unix systems}

Installation on Unix system uses the commonly found {\em configure},
{\em make} and {\em make install} sequence. SWI-Prolog should be
installed before building this package. If SWI-Prolog is not installed
as \program{pl}, the environment variable \env{PL} must be set to the
name of the SWI-Prolog executable. Installation is now accomplished
using:

\begin{code}
% ./configure
% make
% make install
\end{code}

This installs the foreign libraries in \file{$PLBASE/lib/$PLARCH} and
the Prolog library files in \file{$PLBASE/library}, where
\file{$PLBASE} refers to the SWI-Prolog `home-directory'.


\section{Acknowledgments}

The SWI-Prolog ODBC interface started from a partial interface by
Stefano De Giorgi. Mike Elston suggested programmable
null-representation with many other suggestions while doing the first
field-tests with this package.

\bibliographystyle{plain}
\bibliography{odbc}

\printindex

\end{document}