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mysql怎么创建,删除,查看索引?
2022-07-07 10:36:00 【全栈程序员站长】
mysql是一个开源的应用非常广泛的数据库。mysql里面的索引能利用利用指针,能够大大提高查询效率。特别是当数据量非常大,查询涉及多个表时,使用索引往往能使查询速度加快成千上万倍。那么,怎么创建索引呢?
- 第一步,创建一个表,结构如下:create table t (id int unsigned not null,name varchar(20) not null default ‘-‘);
- 第二步,添加索引,二索引有分为几类,普通索引,主键索引,和唯一索引,如图就是步骤:图中数123就是对应普通索引,主键索引,和唯一索引 alter table 表名 add index/unique/primary key (索引名);
- 第三步,表里面已经有索引了,要怎么查看呢?用这两个命令:show index from table;或show keys from table;如图: show index from 表名; show keys from 表名;
- 第四步,有时候索引太多,会引起增删改查的性能,所以可以创建就可以删除,命令如下: drop index 索引名 on 表名; DROP INDEX index_name ON talbe_name; ALTER TABLE table_name DROP INDEX index_name; ALTER TABLE table_name DROP PRIMARY KEY;
- 第五步,在上面删除索引的时候,出现了错误,说不能发现索引名,是因为在第一次的时候已经删除了,所以,此索引名已经不存在了,如图
- 第六步,再次查询索引,是否有没有删除成功,重复第三步的命令,结果如图,说明已经删除了
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