---官方的说明
Key is MUL, multiple occurrences of a given value are allowed within the column. The column is the first column of a non-unique index or a unique-valued index that can contain NULL values.
---自己的测试
mysql> create table t_mul(id int,key1 int,name char(8));
Query OK, 0 rows affected (0.13 sec)
mysql>
mysql> desc t_mul;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| key1 | int(11) | YES | | NULL | |
| name | char(8) | YES | | NULL | |
+-------+---------+------+-----+---------+-------+
3 rows in set (0.01 sec)
mysql>
mysql>
mysql> insert into t_mul values(1,1,'a');
Query OK, 1 row affected (0.01 sec)
mysql> insert into t_mul values(2,1,'a');
Query OK, 1 row affected (0.00 sec)
mysql>
mysql> desc t_mul;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| key1 | int(11) | YES | | NULL | |
| name | char(8) | YES | | NULL | |
+-------+---------+------+-----+---------+-------+
3 rows in set (0.00 sec)
mysql>
mysql> create index i_001 on t_mul(key1,id);
Query OK, 2 rows affected (0.03 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql>
mysql> desc t_mul;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| key1 | int(11) | YES | MUL | NULL | |
| name | char(8) | YES | | NULL | |
+-------+---------+------+-----+---------+-------+
3 rows in set (0.00 sec)
mysql>
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