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create table c2 (num int primary key, t int null);
desc c2;
show create table c2;
alter table c2 modify num integer unique key;
alter table c2 drop primary key;
desc c2;
show create table c2;
Field Type Null Key Default Extra
num int(11) NO PRI null
t int(11) YES null
Table Create Table
c2 CREATE TABLE `c2` (
  `num` int(11) NOT NULL,
  `t` int(11) DEFAULT NULL,
  PRIMARY KEY (`num`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1		 
Records: 0  Duplicates: 0  Warnings: 0

Records: 0  Duplicates: 0  Warnings: 0
Field Type Null Key Default Extra
num int(11) NO PRI 0
t int(11) YES null
Table Create Table
c2 CREATE TABLE `c2` (
  `num` int(11) NOT NULL DEFAULT '0',
  `t` int(11) DEFAULT NULL,
  UNIQUE KEY `num` (`num`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1