MySQL 中的基数是什么？

mysql> create table UniqueDemo1
   -> (
   -> id int,
   -> name varchar(100),
   ->UNIQUE(id,name)
   -> );
Query OK, 0 rows affected (0.81 sec

mysql> insert into UniqueDemo1 values(1,'John');
Query OK, 1 row affected (0.18 sec)

mysql> insert into UniqueDemo1 values(1,'John');
ERROR 1062 (23000): Duplicate entry '1-John' for key 'id'

mysql> select *from UniqueDemo1;

+------+------+
| id   | name |
+------+------+
|    1 | John |
+------+------+
1 row in set (0.00 sec)

mysql> create table LowCardinality
   -> (
   -> id int,
   -> name varchar(100)
   -> );
Query OK, 0 rows affected (0.71 sec)

mysql> insert into LowCardinality values(1,'John');
Query OK, 1 row affected (0.19 sec)

mysql> insert into LowCardinality values(1,'John');
Query OK, 1 row affected (0.14 sec)

mysql> select *from LowCardinality;

+------+------+
| id   | name |
+------+------+
|    1 | John |
|    1 | John |
+------+------+
2 rows in set (0.00 sec)