Oracle主键Primary Key包含非空约束及唯一约束。
添加主键的语句
alter table table_nameadd constraint cons_name primary key(col_name);
查看主键是否被创建成功
select dbms_metadata.get_ddl(‘OBJECT_TYPE‘,‘NAME‘,‘SCHEMA‘) from dual;
OBJECT_TYPE (TABLE,PARTITION,INDEX......)
NAME (OBJECT_NAME)
SCHEMA (INSTANCE)
对应object表中的相应内容：
select object_type,object_name,owner from dba_objects
获取到的DDL语句中表示已经创建了主键列，该列位于CUSTTEL上CREATE TABLE "NEWCCS"."USERLEVELBYTEL_BAK"
( "CUSTTEL" VARCHAR2(20),
"USERLEVEL" NUMBER(*,0) NOT NULL ENABLE,
"CUSTID" VARCHAR2(20)
) PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255 NOCOMPRESS LOGGING
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
TABLESPACE "NEWCCS" ;
ALTER TABLE "NEWCCS"."USERLEVELBYTEL_BAK" MODIFY ("USERLEVEL" NOT NULL ENABLE);
创建主键后发现该表增加了唯一索引及主键字段CREATE TABLE "NEWCCS"."USERLEVELBYTEL_BAK"
( "CUSTTEL" VARCHAR2(20),
"USERLEVEL" NUMBER(*,0) NOT NULL ENABLE,
"CUSTID" VARCHAR2(20),
CONSTRAINT "PK_PRIMARY" PRIMARY KEY ("CUSTTEL")
USING INDEX PCTFREE 10 INITRANS 2 MAXTRANS 255
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
TABLESPACE "NEWCCS" ENABLE
) PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255 NOCOMPRESS LOGGING
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
TABLESPACE "NEWCCS" ;
CREATE UNIQUE INDEX "NEWCCS"."PK_PRIMARY" ON "NEWCCS"."USERLEVELBYTEL_BAK" ("CUSTTEL")
PCTFREE 10 INITRANS 2 MAXTRANS 255
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
TABLESPACE "NEWCCS" ;
ALTER TABLE "NEWCCS"."USERLEVELBYTEL_BAK" ADD CONSTRAINT "PK_PRIMARY" PRIMARY KEY ("CUSTTEL")
USING INDEX PCTFREE 10 INITRANS 2 MAXTRANS 255
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
TABLESPACE "NEWCCS" ENABLE;
ALTER TABLE "NEWCCS"."USERLEVELBYTEL_BAK" MODIFY ("USERLEVEL" NOT NULL ENABLE);
验证：
反复向表中插入数据 insert into userlevelbytel values(‘10010‘,‘3‘,‘ ‘);
出现如下错误提示(唯一约束错误)：
ORA-00001: unique constraint (NEWCCS.PK_PRIMARY) violated
插入空值 insert into userlevelbytel values(‘‘,‘3‘,‘ ‘);
出现如下错误提示(唯一约束错误)：
ORA-01400: cannot insert NULL into ("NEWCCS"."USERLEVELBYTEL_BAK"."CUSTTEL")
单独添加唯一约束对比区别
删除该表主键：
alter table userlevelbytel_bak drop constraint pk_primary;CREATE TABLE "NEWCCS"."USERLEVELBYTEL_BAK"
( "CUSTTEL" VARCHAR2(20),
"USERLEVEL" NUMBER(*,0) NOT NULL ENABLE,
"CUSTID" VARCHAR2(20),
CONSTRAINT "UNIQUE_CUSTTEL" UNIQUE ("CUSTTEL")
USING INDEX PCTFREE 10 INITRANS 2 MAXTRANS 255 COMPUTE STATISTICS
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
TABLESPACE "NEWCCS" ENABLE
) PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255 NOCOMPRESS LOGGING
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
TABLESPACE "NEWCCS" ;
CREATE UNIQUE INDEX "NEWCCS"."UNIQUE_CUSTTEL" ON "NEWCCS"."USERLEVELBYTEL_BAK" ("CUSTTEL")
PCTFREE 10 INITRANS 2 MAXTRANS 255 COMPUTE STATISTICS
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
TABLESPACE "NEWCCS" ;
ALTER TABLE "NEWCCS"."USERLEVELBYTEL_BAK" MODIFY ("USERLEVEL" NOT NULL ENABLE);
ALTER TABLE "NEWCCS"."USERLEVELBYTEL_BAK" ADD CONSTRAINT "UNIQUE_CUSTTEL" UNIQUE ("CUSTTEL")
USING INDEX PCTFREE 10 INITRANS 2 MAXTRANS 255 COMPUTE STATISTICS
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
TABLESPACE "NEWCCS" ENABLE;
可以看到唯一约束被创建
测试：
反复插入非空记录：
ORA-00001: unique constraint (NEWCCS.UNIQUE_CUSTTEL) violated
反复插入空记录：
select * from userlevelbytel_bak where custtel is null;
查询结果为多个重复的空记录
这里面可以看出唯一约束仅对非空值进行约束，而在Oracle中NULL值无法比较，因此即使反复插入，唯一约束也无法限制，但在组合约束条件下，比如(A,B,NULL)则是可以进行比较的。
举例：
alter table userlevelbytel_bak add constraint unique_custtel unique(custtel,userlevel);
反复插入空记录：
ORA-00001: unique constraint (NEWCCS.UNIQUE_CUSTTEL) violated
原文：http://onlinekof2001.blog.51cto.com/3106724/1582962
