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，但如果某個約束作用于多個字段，必須在表級定義約束
    ◆在定義約束時可以通過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。