drop table if exists t1;
create table t1 (a int check (a>0));
insert into t1 values (1);
insert into t1 values (0);
drop table t1;
create table t1 (a int ,b int, check a>b);
insert into t1 values (1,0);
insert into t1 values (0,1);
drop table t1;
create table t1 (a int ,b int, constraint abc check (a>b));
insert into t1 values (1,0);
insert into t1 values (0,1);
drop table t1;
create table t1 (a int null);
insert into t1 values (1),(NULL);
drop table t1;
create table t1 (a int null);
alter table t1 add constraint constraint_1 unique (a);
alter table t1 add constraint unique key_1(a);
alter table t1 add constraint constraint_2 unique key_2(a);
show create table t1;
Table	Create Table
t1	CREATE TABLE `t1` (
  `a` int(11) DEFAULT NULL,
  UNIQUE KEY `constraint_1` (`a`),
  UNIQUE KEY `key_1` (`a`),
  UNIQUE KEY `key_2` (`a`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1
drop table t1;