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);
Table altered.
scott@CNMMBO> drop table tb_dept; -->删除刚刚穿件的表tb_dept
Table 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
