The process of quickly converting mysql left join to inner join

--Class table CREATE TABLE T_CLASS(
  class_id int not null,
  class_name VARCHAR2(100)
);
Add index alter table T_CLASS add index inx_class_id(class_id);

--Student table CREATE TABLE T_STUDENT(
  student_id int not null,
  class_id int not null,
  student_name VARCHAR(100),
  age int,
  sex int 
)
Add index alter table T_STUDENT add index index_age(AGE);

--Insert class data into T_CLASS (CLASS_ID, CLASS_NAME)
values ​​(1, 'Class 1');

insert into T_CLASS (CLASS_ID, CLASS_NAME)
values ​​(2, 'Class 2');

insert into T_CLASS (CLASS_ID, CLASS_NAME)
values ​​(3, 'three shifts');

insert into T_CLASS (CLASS_ID, CLASS_NAME)
values ​​(4, 'fourth shift');

insert into T_CLASS (CLASS_ID, CLASS_NAME)
values ​​(5, 'Fifth Class');

--Insert student data into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)
values ​​(1, 1, '李1', 3, '1');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)
values ​​(2, 1, '李2', 2, '1');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)
values ​​(3, 1, '李3', 3, '1');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)
values ​​(4, 2, '李4', 4, '1');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)
values ​​(5, 2, '李5', 3, '2');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)
values ​​(6, 2, '李6', 3, '1');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)
values ​​(7, 3, '李7', 6, '2');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)
values ​​(8, 3, '李8', 4, '2');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)
values ​​(9, 2, '李9', 2, '2');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)
values ​​(10, 2, '李10', 3, '1');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)
values ​​(11, 3, '李11', 3, '2');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)
values ​​(12, 2, '李12', 8, '2');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)
values ​​(13, 1, '李13', 6, '2');