MySQL 5.5如何删除索引

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mysql> show keys from dept2;
+-------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| dept2 | 0 | PRIMARY | 1 | deptno | A | 4 | NULL | NULL | | BTREE | | |
| dept2 | 1 | idx_dept2_deptno | 1 | deptno | A | 4 | NULL | NULL | | BTREE | | |
| dept2 | 1 | idx_dept2_report_date | 1 | report_date | A | 4 | NULL | NULL | YES | BTREE | | |
+-------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)

方法①

mysql> drop index idx_dept2_deptno on dept2;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> show keys from dept2;
+-------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| dept2 | 0 | PRIMARY | 1 | deptno | A | 4 | NULL | NULL | | BTREE | | |
| dept2 | 1 | idx_dept2_report_date | 1 | report_date | A | 4 | NULL | NULL | YES | BTREE | | |
+-------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)

--方法②

mysql> alter table dept2 drop index idx_dept2_report_date;
Query OK, 0 rows affected (0.05 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> show keys from dept2;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| dept2 | 0 | PRIMARY | 1 | deptno | A | 4 | NULL | NULL | | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)

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