◆在定义约束时可以通过constraint关键字为约束命名，如果没有指定，oracle将自动为约束建立默认的名称定义primary key约束(单个字段）
create table employees (empno number(5) primary key,...) 指定约束名
create table employees (empno number(5) constraint emp_pk primary key,...)
sql>
sql> create table purchase (
2 product_name varchar2(25),
3 quantity number(4,2),
4 purchase_date date,
5 salesperson varchar2(3)
6 );
table created.
sql>
sql> insert into purchase values ('a', 1, '14-jul-03', 'ca');
1 row created.
sql> insert into purchase values ('b', 75, '14-jul-03', 'bb');
1 row created.
sql> insert into purchase values ('c', 2, '14-jul-03', 'ga');
1 row created.
sql> insert into purchase values ('d', 8, '15-jul-03', 'ga');
1 row created.
sql> insert into purchase values ('a', 20, '15-jul-03', 'lb');
1 row created.
sql> insert into purchase values ('b', 2, '16-jul-03', 'ca');
1 row created.
sql> insert into purchase values ('c', 25, '16-jul-03', 'lb');
1 row created.
sql> insert into purchase values ('d', 2, '17-jul-03', 'bb');
1 row created.
sql>
sql> unique product_name
2 from purchase
3 order by product_name;
product_name
-------------------------
a
b
c
d
4 rows selected.
sql>
sql> select distinct product_name
2 from purchase
3 order by product_name;
product_name
-------------------------
a
b
c
d
4 rows selected.
sql>
sql>
sql> drop table purchase;
table dropped.
sql>
oracle 主键和unique索引 有什么区别
主键约束：建立此约束后，对指定列进行唯一约束，并且自动生成一个聚集的索引。外表可以通过外键的形式连接该列。等等...
unique唯一约束：只是对指定列进行唯一的约束。