postgresql/doc/src/sgml/fuzzystrmatch.sgml

<sect1 id="fuzzystrmatch">
 <title>fuzzystrmatch</title>
 
 <para>
  This section describes the fuzzystrmatch module which provides different
  functions to determine similarities and distance between strings.
 </para>

 <sect2>
  <title>Soundex</title>
  <para>
   The Soundex system is a method of matching similar sounding names
   (or any words) to the same code.  It was initially used by the
   United States Census in 1880, 1900, and 1910, but it has little use
   beyond English names (or the English pronunciation of names), and
   it is not a linguistic tool.
  </para>
  <para>
   When comparing two soundex values to determine similarity, the
   difference function reports how close the match is on a scale
   from zero to four, with zero being no match and four being an
   exact match.
  </para>
  <para>
   The following are some usage examples:
  </para>
  <programlisting>
SELECT soundex('hello world!');

SELECT soundex('Anne'), soundex('Ann'), difference('Anne', 'Ann');
SELECT soundex('Anne'), soundex('Andrew'), difference('Anne', 'Andrew');
SELECT soundex('Anne'), soundex('Margaret'), difference('Anne', 'Margaret');

CREATE TABLE s (nm text);

INSERT INTO s VALUES ('john');
INSERT INTO s VALUES ('joan');
INSERT INTO s VALUES ('wobbly');
INSERT INTO s VALUES ('jack');

SELECT * FROM s WHERE soundex(nm) = soundex('john');

SELECT a.nm, b.nm FROM s a, s b WHERE soundex(a.nm) = soundex(b.nm) AND a.oid &lt;&gt; b.oid;

CREATE FUNCTION text_sx_eq(text, text) RETURNS boolean AS
'select soundex($1) = soundex($2)'
LANGUAGE SQL;

CREATE FUNCTION text_sx_lt(text, text) RETURNS boolean AS
'select soundex($1) &lt; soundex($2)'
LANGUAGE SQL;

CREATE FUNCTION text_sx_gt(text, text) RETURNS boolean AS
'select soundex($1) &gt; soundex($2)'
LANGUAGE SQL;

CREATE FUNCTION text_sx_le(text, text) RETURNS boolean AS
'select soundex($1) &lt;= soundex($2)'
LANGUAGE SQL;

CREATE FUNCTION text_sx_ge(text, text) RETURNS boolean AS
'select soundex($1) &gt;= soundex($2)'
LANGUAGE SQL;

CREATE FUNCTION text_sx_ne(text, text) RETURNS boolean AS
'select soundex($1) &lt;&gt; soundex($2)'
LANGUAGE SQL;

DROP OPERATOR #= (text, text);

CREATE OPERATOR #= (leftarg=text, rightarg=text, procedure=text_sx_eq, commutator = #=);

SELECT * FROM s WHERE text_sx_eq(nm, 'john');

SELECT * FROM s WHERE s.nm #= 'john';

SELECT * FROM s WHERE difference(s.nm, 'john') &gt; 2;
  </programlisting>
 </sect2>

 <sect2>
  <title>levenshtein</title>
  <para>
   This function calculates the levenshtein distance between two strings:
  </para>
  <programlisting>
   int levenshtein(text source, text target)
  </programlisting>
  <para>
   Both <literal>source</literal> and <literal>target</literal> can be any
   NOT NULL string with a maximum of 255 characters.
  </para>
  <para>
   Example:
  </para>
  <programlisting>
   SELECT levenshtein('GUMBO','GAMBOL');
  </programlisting>
 </sect2>

 <sect2>
  <title>metaphone</title>
  <para>
   This function calculates and returns the metaphone code of an input string:
  </para>
  <programlisting>
   text metahpone(text source, int max_output_length)
  </programlisting>
  <para>
   <literal>source</literal> has to be a NOT NULL string with a maximum of
   255 characters. <literal>max_output_length</literal> fixes the maximum
   length of the output metaphone code; if longer, the output is truncated 
   to this length.
  </para>
  <para>Example</para>
  <programlisting>
   SELECT metaphone('GUMBO',4);
  </programlisting>
 </sect2>

</sect1>