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Tip of the Day
Language: Enterprise
Expertise: All
Aug 18, 2000

The Difference Between SOUNDEX and DIFFERENCE in SQL Server 7.0

SQL Server's SOUNDEX and DIFFERENCE functions can be used to perform queries based on phonetic comparisons. Although they may share similar purposes, they have some differences.

SOUNDEX takes a character string and modifies it to a four-digit code for use in query comparisons. Characters that aren't alphabetic end the comparison, and vowels are ignored. Therefore, the following query comparing "principal" and "principle" yields the same result:

 SELECT SOUNDEX ('principal'), SOUNDEX 
('principle') ----- ----- P652 P652
But a query comparing the words cat and dog shows these words as different:
 SELECT SOUNDEX ('cat'), SOUNDEX ('dog') 
----- ----- 
C300  D200
In contrast, the DIFFERENCE function uses the SOUNDEX values of the two strings to determine their phonetic similarity. The resulting value is a 0 to 4 rank of the difference between the two strings, where 4 is the best match. So, comparing "principal" to "principle" with the DIFFERENCE function yields the following result:
 SELECT DIFFERENCE('principal', 'principle')

-----
4
But a query looking at the difference between the words cat and dog shows a different result:
 SELECT DIFFERENCE('cat', 'dog')

---- 
2
Andrew Holliday
 
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