SQL优化

use three20210615 ;


create table student(
    studentid varchar(10) ,
    studentname varchar(10),
    gender char ,
    age int ,
    index(studentname)

);

show create table student;
-- 执行计划  explain
insert into student values ('001' ,"潘叔" , '男' , 59 );
select * from student ;
explain select * from student;

select * from student where studentname ="潘叔";
explain select * from student where studentname ="潘叔";


create table student2(
    studentid varchar(10),
    studentname varchar(10),
    age int ,
    city varchar(20),
    dormitoryNo varchar(5),
    index MultiIndex (city , dormitoryNo )
);

insert into student2  values ("002" , "嘎子" , 28  , "河北" , "304");
select * from student2;
explain select * from student2;
select * from student2 where city ="河北" and dormitoryNo  = "304";
explain select * from student2 where city ="河北" and dormitoryNo  = "304";