我们如何从MySQL表中删除唯一约束？

DROP INDEX index_name ON table_name;
OR
ALTER TABLE table_name DROP INDEX index_name;

mysql> Show Index from empl\G
*************************** 1. row ***************************
Table: empl
Non_unique: 0
Key_name: empno
Seq_in_index: 1
Column_name: empno
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
Index_comment:
1 row in set (0.02 sec)

mysql> ALTER TABLE empl DROP INDEX empno;
Query OK, 0 rows affected (0.26 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> describe empl;

+--------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| empno | int(11) | YES | | NULL | |
| F_name | varchar(20) | YES | | NULL | |
+--------+-------------+------+-----+---------+-------+

2 rows in set (0.04 sec)

mysql> Show index from empl;
Empty set (0.00 sec)

mysql> DROP INDEX empno on empl;
Query OK, 0 rows affected (0.17 sec)
Records: 0 Duplicates: 0 Warnings: 0