mysql索引创建命令
1.PRIMARY KEY（主键索引）
mysql>ALTER TABLE admin ADD PRIMARY KEY ( username )
2.UNIQUE(唯一索引)
mysql>ALTER TABLE admin ADD UNIQUE (username )
3.INDEX(普通索引)
mysql>ALTER TABLE admin ADD INDEX index_name ( username )
4.FULLTEXT(全文索引)
mysql>ALTER TABLE admin ADD FULLTEXT ( username )
5.多列索引
mysql>ALTER TABLE admin ADD INDEX index_name ( username, password )
