The fraction of the records that contain unique or non-unique names:
mysql> mysql> CREATE TABLE cat_mailing -> ( -> last_name CHAR(40) NOT NULL, -> first_name CHAR(40) NOT NULL, -> street CHAR(40) NOT NULL -> ); Query OK, 0 rows affected (0.01 sec) mysql> mysql> INSERT INTO cat_mailing (first_name, last_name, street) -> VALUES -> ('Jim','Isaacson','515 Fordam St., Apt. 917'), -> ('Wallace','Baxter','57 3rd Ave.'), -> ('Taylor','McTavish','432 River Run'), -> ('Marlene','Pinter','9 Sunset Trail'), -> ('WALLACE','BAXTER','57 3rd Ave.'), -> ('Bartholomew','Brown','432 River Run'), -> ('Marlene','Pinter','9 Sunset Trail'), -> ('Wallace','Baxter','57 3rd Ave., Apt 102') -> ; Query OK, 8 rows affected (0.00 sec) Records: 8 Duplicates: 0 Warnings: 0 mysql> mysql> SELECT * FROM cat_mailing; +-----------+-------------+--------------------------+ | last_name | first_name | street | +-----------+-------------+--------------------------+ | Isaacson | Jim | 515 Fordam St., Apt. 917 | | Baxter | Wallace | 57 3rd Ave. | | McTavish | Taylor | 432 River Run | | Pinter | Marlene | 9 Sunset Trail | | BAXTER | WALLACE | 57 3rd Ave. | | Brown | Bartholomew | 432 River Run | | Pinter | Marlene | 9 Sunset Trail | | Baxter | Wallace | 57 3rd Ave., Apt 102 | +-----------+-------------+--------------------------+ 8 rows in set (0.00 sec) mysql> mysql> mysql> SELECT COUNT(DISTINCT last_name, first_name) / COUNT(*) -> AS 'unique', -> 1 - (COUNT(DISTINCT last_name, first_name) / COUNT(*)) -> AS 'non-unique' -> FROM cat_mailing; +--------+------------+ | unique | non-unique | +--------+------------+ | 0.6250 | 0.3750 | +--------+------------+ 1 row in set (0.00 sec) mysql> mysql> drop table cat_mailing; Query OK, 0 rows affected (0.00 sec) mysql>