oracle,UNIQUE,/,DISTINCT用法|oracle数据库基本语句

oracle unique约束，在这里拿出来和大家分享一下，希望对大家有用。如果某个约束只作用于单独的字段，即可以在字段级定义约束，也可以在表级定义约束，但如果某个约束作用于多个字段，必须在表级定义约束

◆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> 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。