create table student
(
  name varchar(20) comment '字段的注释',
  age int comment '字段的注释'
)comment='表的注释';

alter table student comment '修改后的表的注释';

alter table student 
modify column name varchar(20) comment '修改后的字段注释';
--注意：字段名和字段类型照写就行

--在生成的SQL语句中看
show create table student;
--在元数据的表里面看
use information_schema;
select * from TABLES where TABLE_SCHEMA='demo' and TABLE_NAME='student' \G

--show
show full columns from student;
--在元数据的表里面看
select * from COLUMNS where TABLE_SCHEMA='demo' and TABLE_NAME='student' \G