mysql> create index agei on student(age desc); //agei是索引名，desc是降序

mysql> create index agei on student(age asc); //asc是默認，升序

2、刪除索引

mysql> alter table student drop index agei;

3、建立索引後，再使用select查詢會增快查詢速度

mysql> select * from student where Age > 18;

4、索引的缺點

占用磁盤空間，降低插入速度和修改速度

5、索引的優點

索引是對數據庫表中一列或多列的值進行排序的一種結構，使用索引可快速訪問數據庫表中的特定信息，加快數據庫的查詢速度

6、插入語句insert

mysql> create table student(

ID int unsigned auto_increment,

Name varchar(10),

Gender enum("男","女"),

Age tinyint unsigned default 18, //如果不寫年齡，默認為18

Score tinyint unsigned,

Address varchar(200),

primary key(ID) );

mysql> insert into student (Name,Gender,Score,Address) values ("張三","男",95,"北京市海澱區"); //插入語句，可以不插入Age字段信息，因為有默認值

7、將查詢結果再插入表中

mysql> insert into student (Name,Gender,Age,Score,Address) select Name,Gender,Age,Score,Address from student;

8、無條件刪除所有記錄

mysql> delete from student; //相當於把表清空

9、replace語句，在插入的時候如果指定主鍵，並且主鍵裏面有插入的值，就替換

mysql> replace into student (ID,Name,Gender,Score,Address) values (1,"張三","男",95,"北京市海澱區");

mysql> replace into student (ID,Name,Gender,Score,Address) values (2,"張三","男",95,"北京市海澱區");

mysql> replace into student (ID,Name,Gender,Score,Address) values (2,"楊貴妃","女",100,"北京市朝陽區"); //次語句會把ID為2的數據給替換掉

簡單索引和insert、replace語句