From United States Census Bureau

“The Soundex is a coded surname index (using the first letter of the last name and three digits) based on the way a name sounds rather than the way it’s spelled. Surnames that sound the same but are spelled differently – such as Smith and Smyth – have the same code and are filed together. This system was developed to make it easier to find a particular name even though it may have been spelled (or misspelled, as was more often the case) a variety of ways.”

“The decennial census Soundex system began as a project of the Franklin D. Roosevelt Administration’s Works Progress Administration (WPA). Because many states did not have uniform systems for registering births, the Soundex indexes were originally prepared to assist the Census Bureau in finding records for people who needed official proof of age.”

SQL Server has two Soundex related functions.

  • SOUNDEX(String)  returns a four character soundex string
  • DIFFERENCE (StringA, StringB) returns a ranking  0 to 4 with 4 being a perfect soundex match.

Here is an excellent post on this subject:

Do You Know Soundex Functions