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>

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The fraction of the records that contain unique or non-unique names: : Distinct « Select Clause « SQL / MySQL

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