create table as

对 DBA 而言，CREATE TABLE AS 可谓是家常便饭，顺手拈来。需不知该方式虽然简单，但疏忽也容易导致意想不到的问题。笔者前阵子就碰上
了这样的事情。由于是对原表进行克隆，且数据存储在不同的表空间，因此毫不犹豫地使用了CREATE TABLE AS，结果在运行package时，error...

 --1、非空约束遗失-->使用create table as 来创建对象scott@CNMMBO> create table tb_dept as select * from dept where 1=0;Table created.scott@CNMMBO> desc dept; Name                                                  Null?    Type ----------------------------------------------------- -------- ------------------------------------ DEPTNO                                                NOT NULL NUMBER(2) DNAME                                                          VARCHAR2(14) LOC                                                            VARCHAR2(13)scott@CNMMBO> desc tb_dept; Name                                                  Null?    Type ----------------------------------------------------- -------- ------------------------------------ DEPTNO                                                         NUMBER(2) DNAME                                                          VARCHAR2(14) LOC                                                            VARCHAR2(13)-->从上面的desc可以看出新创建的表少了非空约束-->下面手动为其增加非空约束，增加后与原来的表是一致的。当然使用create table as时，索引是需要单独重建的。scott@CNMMBO> alter table tb_dept modify (deptno not null);--只需要描述待修改的属性即可，不变属性不用再次描述 Table altered.scott@CNMMBO> drop table tb_dept;    -->删除刚刚穿件的表tb_deptTable dropped.--2、存在非空约束时default约束遗失-->下面为表dept的loc列添加非空约束，且赋予default值scott@CNMMBO> alter table dept modify (loc default 'BeiJing' not null);Table altered.-->为原始表新增一条记录scott@CNMMBO> insert into dept(deptno,dname) select 50,'DEV' from dual;1 row created.scott@CNMMBO> commit;Commit complete.-->下面的查询可以看到新增记录50的loc为缺省值'BeiJing'scott@CNMMBO> select * from dept;    DEPTNO DNAME          LOC---------- -------------- -------------        10 ACCOUNTING     NEW YORK        20 RESEARCH       DALLAS        30 SALES          CHICAGO        40 OPERATIONS     BOSTON        50 DEV            BeiJing-->再次使用create table as来创建对象scott@CNMMBO> create table tb_dept as select * from dept;Table created.        -->从下面可知，由于列loc存在default值，所以此时not null约束被同时赋予scott@CNMMBO> desc tb_dept Name                                                  Null?    Type ----------------------------------------------------- -------- ------------------------------------ DEPTNO                                                         NUMBER(2) DNAME                                                          VARCHAR2(14) LOC                                                   NOT NULL VARCHAR2(13) scott@CNMMBO> select * from tb_dept;    DEPTNO DNAME          LOC---------- -------------- -------------        10 ACCOUNTING     NEW YORK        20 RESEARCH       DALLAS        30 SALES          CHICAGO        40 OPERATIONS     BOSTON        50 DEV            BeiJing -->为新创建的表新增记录-->新增时发现尽管not null约束生效，但原表上设定的default值不存在了scott@CNMMBO> insert into tb_dept(deptno,dname) select 60,'HR' from dual;insert into tb_dept(deptno,dname) select 60,'HR' from dual*ERROR at line 1:ORA-01400: cannot insert NULL into ("SCOTT"."TB_DEPT"."LOC")scott@CNMMBO> drop table tb_dept;Table dropped.--3、唯一约束遗失scott@CNMMBO> alter table dept modify (dname unique);Table altered.scott@CNMMBO> create table tb_dept as select * from dept;Table created.scott@CNMMBO> insert into tb_dept select 60,'DEV','ShangHai' from dual;1 row created.scott@CNMMBO> commit;Commit complete.scott@CNMMBO> select * from tb_dept;    DEPTNO DNAME          LOC---------- -------------- -------------        10 ACCOUNTING     NEW YORK        20 RESEARCH       DALLAS        30 SALES          CHICAGO        40 OPERATIONS     BOSTON        50 DEV            BeiJing        60 DEV            ShangHai-->有关check约束与外键约束不再演示--4、最彻底的解决办法scott@CNMMBO> select dbms_metadata.get_ddl('TABLE','DEPT') from dual;DBMS_METADATA.GET_DDL('TABLE','DEPT')--------------------------------------------------------------------------------  CREATE TABLE "SCOTT"."DEPT"   (    "DEPTNO" NUMBER(2,0),        "DNAME" VARCHAR2(14),        "LOC" VARCHAR2(13) DEFAULT 'BeiJing' NOT NULL ENABLE,         CONSTRAINT "PK_DEPT" PRIMARY KEY ("DEPTNO")  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 "GOEX_USERS_TBL"  ENABLE,         UNIQUE ("DNAME")  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 "GOEX_USERS_TBL"  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 "GOEX_USERS_TBL"--5、演示环境scott@CNMMBO> select * from v$version where rownum<2;BANNER----------------------------------------------------------------Oracle Database 10g Release 10.2.0.3.0 - 64bit Production--6、演示结论-->create table as 尽管会克隆表及数据，数据是完整的，但是结构部分仅仅是部分克隆-->create table as 会使用表上的约束被遗失或出于非正常状态-->create table as 时，表上的索引、触发器等不会被同时克隆-->create table as 仅作测试使用，要得到完整的结构语句，还是使用dbms_metadata.get_ddl包-->Author: Robinson Cheng            -->Blog:   http://blog.csdn.net/robinson_0612