Support AlTER TABLE MODIFY COLUMN.

[qiuyesuifeng]

drop table if exists product;

create table product (
productid varchar(10) not null,
category varchar(10) not null,
name varchar(80) null,
descn varchar(255) null,
constraint pk_product primary key (productid)
);

ALTER TABLE product MODIFY COLUMN name varchar(80);   

[coocood]

Duplicated to #1753