sql example 4 – alter

• character
  

  show create database test;
  alter database test character set utf8;
  alter database test character set gbk;
  

  show create table user1;
  alter table user1 character set gbk;
  alter table user1 character set utf8;
  

• add
  

  alter table user1 add age int not null default 10;           # 添加到最后一列
  alter table user1 add age1 int not null default 10 first;    # 添加到第一列
  alter table user1 add age2 int not null default 10 after age;
  alter table user1 add age3 int not null, add age4 int not null;
  

  alter table user1 drop age;
  alter table user1 drop age1, drop age2, drop age3, drop age4;
  

• primary key and foreign key
  

  create table test1 (
  id int ,
  username varchar(10) not null
  );
  
  create table test2 (
  id2 int not null,
  username2 varchar(10) not null
  );
  

  primary key:
  

  alter table test2 add constraint pk_test2_id primary key(id2);
  
  show columns from test2;
  
  alter table test2 drop primary key;
  
  show columns from test2;
  

  创建 foreign key:
  

  alter table test2 add foreign key (id2) references test1(id);      # 类型要相同
  
  Can‘t create table ‘test.#sql-158e_11‘ (errno: 150)                # 类型不同 150
  
  alter table test2 modify id2 int not null;
  alter table test1 modify id int not null;
  alter table test1 add foreign key (id) references test2(id2);
  
  Can‘t create table ‘test.#sql-158e_11‘ (errno: 150)
  alter table test2 add constraint pk_test2_id primary key(id2);      # 父表一定要是主键
  alter table test1 add foreign key (id) references test2(id2);
  show columns from test2;
  

  go to modify
  

  删除 foreign key:
  

  show keys from test1;
  show index from test1;
  show create table test1;
                  #  test1_ibfk_1
  alter table test2 drop foreign key test1_ibfk_1;
  

• default
  

  alter table test2 add age int not null ;
  alter table test2 alter age set default 15;    # 添加 default 属性
  alter table test2 alter age drop default;      # 删除 default 属性
  

  alter table test2 add unique (username2);
  show columns from test2;
  

• modify go back
  

  alter table test2 modify id smallint not null FIRST;
  alter table test2 modify id tinyint not null FIRST;   # 把 *大整数* 切换到 *小整数* 注意防止数据丢失
  
  alter table test2 change pid p_id tinyint;
  alter table test2 change p_id pid int;
  

• rename, 一般不能更改, 乱改数据可能出错
  

  rename table test1 to test3;
  alter table test3 rename to test1;
  
  drop table test1, test2;