alter table的操作有增加列、删除列和修改列的长度等

create table t1 (c1 int primary key) engine = innodb;
// 增加一个列c2
alter table t1 add c2 varchar(16379);
drop table t1;create table t1 (c1 int primary key, c2 varchar(50)) engine = innodb;
// 将列c2长度更改
alter table t1 modify column c2 varchar(16234);
drop table t1;// 删除一个列c2
create table t1 (c1 int primary key, c2 varchar(50)) engine = innodb;
alter table t1 drop column c2;
drop table t1;