oracle unique約束，在這里拿出來和大家分享一下，希望對大家有用。如果某個(gè)約束只作用于單獨(dú)的字段，即可以在字段級定義約束，也可以在表級定義約束，但如果某個(gè)約束作用于多個(gè)字段，必須在表級定義約束

◆在定義約束時(shí)可以通過constraint關(guān)鍵字為約束命名，如果沒有指定，oracle將自動為約束建立默認(rèn)的名稱定義primary key約束(單個(gè)字段)

create table employees (empno number(5) primary key,...) 指定約束名

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> select 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 主鍵和unique索引 有什么區(qū)別

主鍵約束：建立此約束后，對指定列進(jìn)行唯一約束，并且自動生成一個(gè)聚集的索引。外表可以通過外鍵的形式連接該列。等等...

unique唯一約束：只是對指定列進(jìn)行唯一的約束。

更多信息請查看IT技術(shù)專欄