可以分为两步，先取消掉自增，再改字段类型

create table ttt(id int IDENTITY(1,1) PRIMARY KEY,name VARCHAR(20));
alter table ttt drop IDENTITY;
alter table ttt MODIFY id VARCHAR(20);

-- 修改后的DDL语句
CREATE TABLE "SYSDBA"."TTT"
(
"ID" VARCHAR(20) NOT NULL,
"NAME" VARCHAR(20),
NOT CLUSTER PRIMARY KEY("ID")) STORAGE(ON "MAIN", CLUSTERBTR) ;