fuzzystrmatch
fuzzystrmatch
The fuzzystrmatch> module provides several
functions to determine similarities and distance between strings.
At present, the soundex>, metaphone>,
dmetaphone>, and dmetaphone_alt> functions do
not work well with multi-byte encodings (such as UTF-8).
Soundex
The Soundex system is a method of matching similar-sounding names
by converting them to the same code. It was initially used by the
United States Census in 1880, 1900, and 1910. Note that Soundex
is not very useful for non-English names.
The fuzzystrmatch> module provides two functions
for working with Soundex codes:
soundex(text) returns text
difference(text, text) returns int
The soundex> function converts a string to its Soundex code.
The difference> function converts two strings to their Soundex
codes and then reports the number of matching code positions. Since
Soundex codes have four characters, the result ranges from zero to four,
with zero being no match and four being an exact match. (Thus, the
function is misnamed — similarity> would have been
a better name.)
Here are some usage examples:
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 * FROM s WHERE difference(s.nm, 'john') > 2;
Levenshtein
This function calculates the Levenshtein distance between two strings:
levenshtein(text source, text target, int ins_cost, int del_cost, int sub_cost) returns int
levenshtein(text source, text target) returns int
Both source and target can be any
non-null string, with a maximum of 255 bytes. The cost parameters
specify how much to charge for a character insertion, deletion, or
substitution, respectively. You can omit the cost parameters, as in
the second version of the function; in that case they all default to 1.
Examples:
test=# SELECT levenshtein('GUMBO', 'GAMBOL');
levenshtein
-------------
2
(1 row)
test=# SELECT levenshtein('GUMBO', 'GAMBOL', 2,1,1);
levenshtein
-------------
3
(1 row)
Metaphone
Metaphone, like Soundex, is based on the idea of constructing a
representative code for an input string. Two strings are then
deemed similar if they have the same codes.
This function calculates the metaphone code of an input string:
metaphone(text source, int max_output_length) returns text
source has to be a non-null string with a maximum of
255 characters. max_output_length sets the maximum
length of the output metaphone code; if longer, the output is truncated
to this length.
Example:
test=# SELECT metaphone('GUMBO', 4);
metaphone
-----------
KM
(1 row)
Double Metaphone
The Double Metaphone system computes two sounds like> strings
for a given input string — a primary> and an
alternate>. In most cases they are the same, but for non-English
names especially they can be a bit different, depending on pronunciation.
These functions compute the primary and alternate codes:
dmetaphone(text source) returns text
dmetaphone_alt(text source) returns text
There is no length limit on the input strings.
Example:
test=# select dmetaphone('gumbo');
dmetaphone
------------
KMP
(1 row)