1.create table 表名 (属性名 数据类型 unique);
for example : create table t_dept (
                    deptno int,
                    dname varchar(20) unique,
                    loc varchar(20));
2. create table t_dept(
                     deptno int,
                     dname varchar(20),
                     loc varchar(20),
                     constraint uk_dname unique(dname));
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