This section describes the predicates required for creating and destroying the access to external database tables.
- new_table(+File, +Columns, +Options, -Handle)
- Create a description of a new table, stored in File. Columns
is a list of descriptions for each column. A column description is of
(Type [, ColumnOptions]
Type denotes the Prolog type to which the field should be converted and is one of:
Convert to a Prolog integer. The input is treated as a decimal number.
Convert to a Prolog integer. The input is treated as a hex number.
Convert to a Prolog floating point number. The input is handled by the C-library function
Convert to a Prolog atom.
Convert to a SWI-Prolog string object.
Convert to a list of ASCII codes.
ColumnOptions is a list of additional properties of the column. Supported values are:
The field is strictly sorted, but may have (adjacent) duplicate entries. If the field is textual, it should be sorted alphabetically, otherwise it should be sorted numerically.
The (textual) field is sorted using the ordering declared by the named ordering table. This option may be used to define reverse order, `dictionary' order or other irregular alphabetical ordering. See new_order_table/2.
This column has distinct values for each row in the table.
Map all uppercase in the field to lowercase before converting to a Prolog atom, string or code_list.
Map spaces to underscores before converting to a Prolog atom, string or code_list.
For numerical fields. If the field does not contain a valid number, matching the value fails. Reading the value returns the value as an atom.
Field has fixed width of the specified number of characters. The column-separator is not considered for this column.
For read_table_record/4, unify the field with the given argument of the record term. Further fields will be assigned index+1, ... .
Don't convert this field to Prolog. The field is simply skipped without checking for consistency.
The Options argument is a list of global options for the table. Defined options are:
Character (ASCII) value of the character separating two records. Default is the newline (ASCII 10).
Character (ASCII) value of the character separating two fields in a record. Default is the space (ASCII 32), which also has a special meaning. Two fields separated by a space may be separated by any non-empty sequence of spaces and tab (ASCII 9) characters. For all other separators, a single character separates the fields.
Text encoding of the file. Values are
native. The latter uses the native multibyte to unicode conversion.
Sometimes, a table defines escape sequences to make it possible to use the separator-characters in text-fields. This options provides a simple way to handle some standard cases. Code is the ASCII code of the character that leads the escape sequence. The default is
-1, and thus never matched. ListOfMap is a list of From
=To character mappings. The default map table is the identity map, unless Code refers to the
\character, in which case
\thave their usual meaning.
Functor used by read_table_record/4. Default is
recordusing the maximal argument index of the fields as arity.
If the options are parsed successfully, Handle is unified with a term that may be used as a handle to the table for future operations on it. Note that new_table/4 does not access the file system, so its success only indicates the description could be parsed, not the presence, access or format of the file.
- Open the table. This predicate normally does not need to be called explicitely, as all operations on the table handle will automatically open the table if this is required. It fails if the file cannot be accessed or some other error with the required operating-system resources occurs. The contents of the file is not examined by this predicate.
- Close the file and other system resources, but do not remove the description of the table, so it can be re-opened later.
- Close and remove the handle. After this operation, Handle
becomes invalid and further references to it causes undefined behaviour.