Returns a character string that represents the Soundex code for string_expression.
Soundex
Soundex is a system that codes surnames having the same or similar sounds, but variant spellings. The Soundex system was first used by the National Archives in 1880 to index the United States census.
Soundex codes begin with the first letter of the surname followed by a three-digit code. Zeros are added to names that do not have enough letters.