- Reference manual
- SWI-Prolog binding to libarchive
- SWI-Prolog BerkeleyDB interface
- SWI-Prolog C-library
- Constraint Query Language A high level interface to SQL databases
- SWI-Prolog HTTP support
- SWI-Prolog binding to BSD libedit
- SWI-Prolog Natural Language Processing Primitives
- SWI-Prolog ODBC Interface
- Paxos -- a SWI-Prolog replicating key-value store
- SWI-Prolog Regular Expression library
- SWI-Prolog support for PDT
- Pengines: Web Logic Programming Made Easy
- A C++ interface to SWI-Prolog
- SWI-Prolog Source Documentation Version 2
- Prolog Unit Tests
- Google's Protocol Buffers Library
- SWI-Prolog RDF parser
- SWI-Prolog binding to GNU readline
- SWI-Prolog Semantic Web Library 3.0
- SWI-Prolog SGML/XML parser
- SWI-Prolog SSL Interface
- Managing external tables for SWI-Prolog
- Transparent Inter-Process Communications (TIPC) libraries
- SWI-Prolog Unicode library
- SWI-Prolog YAML library
- SWI-Prolog binding to zlib
University of Amsterdam
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.
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.
The ODBC interface deals with a single ODBC environment with multiple simultaneous connections. The predicates in this section deal with connection management.
- odbc_connect(+DSN, -Connection, +Options)
- Create a new ODBC connection to data-source DSN and return a
handle to this connection in Connection. The connection
handle is either an opaque structure or an atom of the
aliasoption is used. In addition to the options below, options applicable to odbc_set_connection/2 may be provided.
- Define the user-name for the connection. This option must be present if the database uses authorization.
- Provide a password for the connection. Normally used in combination with
- Use AliasName as Connection identifier, making the connection available as a global resource. A good choice is to use the DSN as alias.
- If OpenMode is
once(default if an
aliasis provided), a second call to open the same DSN simply returns the existing connection. If
multiple(default if there is no alias name), a second connection to the same data-source is opened.
true, use Microsoft SQL server 2005 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.
- Select the version of the ODBC connection. Default is
'3.0'. The other supported value is
The following example connects to the WordNet1An SQL version of WordNet is available from http://wordnet2sql.infocity.cjb.net/  database, using the connection alias
wordnetand opening the connection only once:
open_wordnet :- odbc_connect('WordNet', _, [ user(jan), password(xxx), alias(wordnet), open(once) ]).
- odbc_driver_connect(+DriverString, -Connection, +Options)
- Connects to a database using SQLDriverConnect(). This API allows for
driver-specific additional options. DriverString is passed without
checking. Options should not include
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.bugFacilities to deal with prompted completion of the driver options are not yet implemented.
- Close the given Connection. This destroys the connection alias or, if there is no alias, makes further use of the Connection handle illegal.
- odbc_current_connection(?Connection, ?DSN)
- Enumerate the existing ODBC connections.
- odbc_set_connection(+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:
read, tell the driver we only access the database in read mode. If
update(default), tell the driver we may execute update commands.
true(default), each update statement is committed immediately. If
false, an update statement starts a transaction that can be committed or rolled-back. See section 2.3 for details on transaction management.
- I haven't found a good description of what this does, but setting it to
dynamicmakes it possible to have multiple active statements on the same connection with Microsoft SQL server. Other values are
- Define the encoding used to communicate to the driver. Defined values
are given below. The default on MS-Windows is
unicodewhile on other platforms it is
utf8. Below, the *A() functions refer to the `ansi' ODBC functions that exchange bytes and the *W() functions refer to the `unicode' ODBC functions that exchange UCS-2 characters.
- Communicate using the *A() functions and pass bytes untranslated.
- Communicate using the *A() functions and translated between Prolog Unicode characters and their (possibly) multibyte representation in the current locale.
- Communicate using the *A() functions and translated between Prolog Unicode characters and their UTF-8 encoding.
- Communicate using the *W() functions.
false), statements returning
SQL_SUCCESS_WITH_INFOsucceed without printing the info. See also section 2.7.1.
- Defines how the SQL constant NULL is represented. Without specification,
the default is the atom
$null$. 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
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.
- If the width of a column exceeds 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.
- odbc_get_connection(+Connection, ?Property)
- Query for properties of the connection. Property is a term of
Name(Value). If Property is unbound all defined properties are enumerated on backtracking. Currently the following properties are defined.
- Name of the database associated to the connection.
- Name of the database engine. This constant can be used to identify the engine.
- Version identifier from the database engine.
- ODBC Dynamic Link Library providing the interface between ODBC and the database.
- ODBC version supported by the driver.
- The drivers version identifier.
- Maximum number of statements that can be active at the same time on this
connection. Returns 0 (zero) if this is unlimited.2Microsoft
SQL server can have multiple active statements after setting the option
dynamic. See odbc_set_connection/2.
- odbc_data_source(?DSN, ?Description)
- Query the defined data sources. It is not required to have any open connections before calling this predicate. DSN is the name of the data source as required by odbc_connect/3. 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.
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.
- odbc_query(+Connection, +SQL, -RowOrAffected)
- Same as odbc_query/4
- odbc_query(+Connection, +SQL, -RowOrAffected, +Options)
- Fire an SQL query on the database represented by Connection.
SQL is any valid SQL statement. SQL statements can be
specified as a plain atom, string or a term of the format
Format-Arguments, which is converted using format/2.
If the statement is a
SELECTstatement the result-set is returned in RowOrAffected. By default rows are returned one-by-one on backtracking as terms of the functor
row/Arity, where 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
findall/2(see below) the result-set is returned as a list of user-specified terms. For other statements this argument returns
affected(Rows), where 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 `
lemma(Lemma) :- odbc_query(wordnet, 'SELECT (lemma) FROM word', row(Lemma)).
The following example adds a name to a table with parent-relations, returning the number of rows affected by the statement. Note that the SQL quote character is the ASCII single quote and, as this SQL quote is embedded in a single quoted Prolog atom, it must be written as
''(two single quotes). We use the first alternative for better visibility.
insert_child(Child, Mother, Father, Affected) :- odbc_query(parents, 'INSERT INTO parents (name,mother,father) \ VALUES (\'mary\', \'christine\', \'bob\')', affected(Affected)).
Options defines the following options.
- Determine the Prolog type used to report the column-values. When
omitted, default conversion as described in section
2.6 is implied. A column may specify
defaultto 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
wordthe first column is defined with the SQL type
DECIMAL(6). Using this SQL-type, ``001'' is distinct from ``1'', but using Prolog integers is a valid representation for Wordnet
wordnoidentifiers. The following query extracts rows using Prolog integers:
?- odbc_query(wordnet, 'select * from word', X, [ types([integer,default]) ]). X = row(1, entity) ; X = row(2, thing) ; ...
See also section 2.6 for notes on type-conversion.
- Specify SQL NULL representation. See odbc_set_connection/2 for details.
false), include the source-column with each result-value. With this option, each result in the
row/N-term is of the format below. TableName or ColumnName may be the empty atom if the information is not available.3This 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.
column(TableName, ColumnName, Value)
- findall(Template, row(Column, ... ))
- 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
lemmas(Lemmas) :- findall(Lemma, odbc_query(wordnet, 'select (lemma) from word', row(Lemma)), Lemmas).
findall/2option the above can be implemented as below. The number of argument of the
rowterm must match the number of columns in the result-set.
lemmas(Lemmas) :- odbc_query(wordnet, 'select (lemma) from word', Lemmas, [ findall(Lemma, row(Lemma)) ]).
The current implementation is incomplete. It does not allow arguments of
row(...)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.
- Specify threshold column width for using SQLGetData(). See odbc_set_connection/2 for details.
- odbc_query(+Connection, +SQL)
- As odbc_query/3,
but used for SQL-statements that should not return result-rows (i.e. all
statements except for
SELECT). The predicate prints a diagnostic message if the query returns a result.
ODBC provides for `parameterized queries'. These are SQL queries with
-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:
- odbc_prepare(+Connection, +SQL, +Parameters, -Statement)
- As odbc_prepare/5
- odbc_prepare(+Connection, +SQL, +Parameters, -Statement, +Options)
- Create a statement from the given SQL (which may be a format
specification as described with odbc_query/3)
statement that normally has one or more parameter-indicators (
) and unify Statement with a handle to the created statement. Parameters is a list of descriptions, one for each parameter. Each parameter description is one of the following:
- Uses the ODBC function SQLDescribeParam() to obtain information about
the parameter and apply default rules. See section
2.6 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
silent(true)option of odbc_set_connection/2. An alternative mapping can be selected using the > option of this predicate described below.
- SqlType(Specifier, ...)
- Declare the parameter to be of type SqlType with the given
specifiers. Specifiers are required for
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 section 2.6 for details.
- PrologType > SqlType
- As above, but supply values of the given PrologType, using
the type-transformation defined by the database driver. For example, if
the parameter is specified as
atom > date
The use must supply an atom of the format
YYYY-MM-DDrather than a term
date(Year,Month,Day). This construct enhances flexibility and allows for passing values that have no proper representation in Prolog.
Options defines a list of options for executing the statement. See odbc_query/4 for details. In addition, the following option is provided:
- Determine the FetchType, which is one of
auto(default) to extract the result-set on backtracking or
fetchto prepare the result-set to be fetched using odbc_fetch/3.
- odbc_execute(+Statement, +ParameterValues, -RowOrAffected)
- Execute a statement prepared with odbc_prepare/4
with the given
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.4Is 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
age (name char(25), age integer)bound to the predicate 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.5The code is prepared to maintain a cache of statements. Practice should tell us whether it is worthwhile activating this.
same_age(X, Y) :- age(X, AgeX), age(Y, AgeY), AgeX = AgeY.
- odbc_execute(+Statement, +ParameterValues)
- Like odbc_query/2, this predicate is meant to execute simple SQL statements without interest in the result.
- If the thread ThreadId is currently blocked inside odbc_execute/4 then interrupt it. If ThreadId is not currently executing odbc_execute/4 then odbc_cancel_thread/1 succeeds but does nothing. If ThreadId is not a valid thread ID or alias, an exception is raised.
- 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.
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.
- odbc_fetch(+Statement, -Row, +Option)
- Fetch a row from the result-set of Statement. Statement
must be created with odbc_prepare/5
using the option
fetch(fetch)and be executed using odbc_execute/2. Row is unified to the fetched row or the atom
end_of_file6This 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. Option is one of:
- Fetch the next row.
- Fetch the result-set going backwards.
- Fetch the first row.
- Fetch the last row.
- Fetch absolute numbered row. Rows count from one.
- Fetch relative to the current row.
relative(1)is the same as
next(), except that the first row extracted is row 2.
- Reserved. Bookmarks are not yet supported in this interface.
In many cases, depending on the driver and RDBMS, the cursor-type must be changed using odbc_set_connection/2 for anything different from
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.
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) ).
- loses the given statement (without freeing it). This must be used if not the whole result-set is retrieved using odbc_fetch/3.
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.
- odbc_end_transaction(+Connection, +Action)
- End the currently open transaction if there is one. Using Action
commitpending updates are made permanent, using
rollbackthey are discarded.
The ODBC documentation has many comments on transaction management and its interaction with database cursors.
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.
- odbc_current_table(+Connection, -Table)
- Return on backtracking the names of all tables in the database identified by the connection.
- odbc_current_table(+Connection, ?Table, ?Facet)
- Enumerate properties of the tables. Defines facets are:
- These facets are defined by SQLTables()
- This facet returns the number of columns in a table.
- odbc_table_column(+Connection, ?Table, ?Column)
- On backtracking, enumerate all columns in all tables.
- odbc_table_column(+Connection, ?Table, ?Column, ?Facet)
- Provides access to the properties of the table as defined by the ODBC
call SQLColumns(). Defined facets are:
- See odbc_current_table/3.
- These facets are defined by SQLColumns()
- More prolog-friendly representation of the type properties. See section 2.6.
- odbc_type(+Connection, ?TypeSpec, ?Facet)
- Query the types supported by the data source. TypeSpec is
either an integer type-id, the name of an ODBC SQL type or the constant
all_typesto enumerate all known types. This predicate calls SQLGetTypeInfo() and its facet names are derived from the specification of this ODBC function:
- Name used by the data-source. Use this in CREATE statements
- Numeric identifier of the type
- When available, maximum precision of the type.
- When available, prefix for literal representation.
- When available, suffix for literal representation.
- When available, arguments needed to create the type.
- Whether the type can be
NULL. May be
- Whether values for this type are case-sensitive.
- Whether the type can be searched. Values are
- When available, whether the value is signed. Please note that SWI-Prolog does not provide unsigned integral values.
- Whether the type represents money.
- When available, whether the type can be auto-incremented.
- Name of the type in local language.
- Minimum scale of the type.
- Maximum scale of the type.
- odbc_table_primary_key(+Connection, +Table, ?Column)
- True when Column is a primary key in Table.
- odbc_table_foreign_key(+Connection, ?PkTable, ?PkCol, ?FkTable, ?FkCol)
- True when PkTable/PkCol FkTable/FkCol is a foreign keys column.
- Get statistical data on the ODBC interface. Currently defined keys are:
- statements(Created, Freed)
- Number of SQL statements that have been Created and Freed over all connections. Statements executed with odbc_query/[2-3] increment Created as the query is created and 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.
- Set the verbosity-level to Level. Default is 0. Higher levels make the system print debugging messages.
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
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
types(TypeList) option for the
- Used as default for the SQL types
numeric. Can be used for all types.
- SWI-Prolog extended type string. Use the type for special cases where garbage atoms must be avoided. Can be used for all types.
- 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.
- Used as default for the SQL types
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
decimaltypes as well as the types
timestamp, which are represented as POSIX time-stamps (seconds after Jan 1, 1970).
- Used as default for the SQL types
double. Can be used for the integral and
decimaltypes as well as the types
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.
- A Prolog term of the form
date(Year,Month,Day)used as default for the SQL type
- A Prolog term of the form
time(Hour,Minute,Second)used as default for the SQL type
- A Prolog term of the form
timestamp(Year,Month,Day,Hour,Minute,Second,Fraction)used as default for the SQL type
ODBC operations return success, error or `success with info'. This section explains how results from the ODBC layer are reported to Prolog.
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
informational and the term is
of the form:
- odbc(State, Native, Message)
- Here, State is the SQL-state as defined in the ODBC API, Native is the (integer) error code of the underlying data source and Message is a human readable explanation of the message.
If an ODBC operation signals an error, it throws the exception
arguments of the
odbc(State, Native, Message), _)
odbc/3 term are explained in section
In addition, the Prolog layer performs the normal tests for proper arguments and state, signaling the conventional instantiation, type, domain and resource exceptions.
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 PHP documentation on this subject, provided by Adabas D, IBM DB2, Solid, and Sybase SQL Anywhere.
On MS-Windows, the ODBC interface is a standard package, linked
The following issues are identified and waiting for concrete problems and suggestions.
- 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.
- High-level interface
- Attaching tables to predicates, partial DataLog implementation, etc.
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.
- George Miller. Wordnet: An on-line lexical database.
International Journal of Lexicography, 3(4), 1990. (Special
- 2.2.1 2.2.1
- 2.1 2.1 2.2.1
- 2.2.2 2.2.2
- 2.2.2 2.2.2
- 2.2.2 2.2.3 2.2.3
- 2.2.2 2.2.2
- 2.2.2 2.2.3 2.6
- 2.2.1 2.2.2
- 2.2.1 2.2.1 2.2.1 2.2.2
- 2.1 2.2.1 2.2.2 2.2.2 2.6
- 2.1 2.1 2.2.1 2.2.1 2.2.2 2.2.3 2.3