Oracle主键及约束

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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_name add 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

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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

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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;

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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