text: --add/change/drop enum column using alter table ¡¦ statement.
  
  
  create table atr(a int);
  insert into atr values(1), (2), (3);
  
  
  --TEST: [er] add an enum column, without enumeration values.
  alter table atr add column col1 enum;
  alter table atr add column col1 enum();
  
  --TEST: add an enum column
  alter table atr add column col1 enum('a', 'b', 'c');
  show columns in atr;
  --TEST: [er] add an enum column, with default value
  alter table atr add column col2 enum('a', 'b', 'c') default 'b';
  --TEST: [er] add an enum column, with shared value
  alter table atr add column col3 enum('a', 'b', 'c') shared 3;
  --TEST: [er] add an enum column, with not null constraint
  alter table atr add column col4 enum('aaa', 'bbb', 'ccc', 'ddd') not null;
  --TEST: [er] add an enum column, with unique constraint
  alter table atr add column col5 enum('a', 'b') unique after col4;
  --TEST: [er] add an enum column, with pk constraint
  alter table atr add column col6 enum('aaa', 'bbb', 'ccc') primary key;
  
  
  --TEST: check the alter result
  show columns in atr;
  
  --TEST: insert data after alter table.
  insert into atr values(4, 'b', 2, 1, 'bbb');
  --TEST: [er] unique violation error
  insert into atr values(4, 'b', 2, 1, 'bbb');
  
  --TEST: check the insert result
  select * from atr order by 1;
  
  
  --TEST: add multiple columns
  alter table atr add column (col7 enum('a', 'b', 'c') unique, col8 enum('aa', 'bb'), col9 enum('aaa', 'ccc') not null);
  show columns in atr;
  
  --TEST: insert after alter
  insert into atr values(5, 'c', 2, 2, 'ccc', 3, 2, 1);
  select * from atr order by 1;
  
  
  drop table atr;
  
  