程序媛计划——mysql索引

Posted 2020-10-06 IcarusYu

定义：

　　索引是一种单独的、物理的对数据库表中一列或多列的值进行排序的一种存储结构

 

#为字段创建索引

#在表中的字段中创建索引
mysql> create index ind_score on score(name);
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0
#查看索引
mysql> show index from score;
+-------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name  | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| score |          0 | PRIMARY   |            1 | id          | A         |           4 |     NULL | NULL   |      | BTREE      |         |               |
| score |          1 | ind_score |            1 | name        | A         |           4 |     NULL | NULL   |      | BTREE      |         |               |
+-------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)

#也可以在创建表的时候创建索引

 

#删除索引

mysql> drop index ind_score on score;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

#用alter创建／删除索引

mysql> alter table score add index ind_score(name);
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table score drop index ind_score;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

 

#创建索引值唯一的索引（好处是数据很多时能通过唯一索引快速查询到数据）

mysql> create unique index uni_index on score(score);
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0