對 DBA 而言，CREATE TABLE AS 可謂是家常便飯，順手拈來。需不知該方式雖然簡單，但疏忽也容易導致意想不到的問題。筆者前陣子就碰上了這樣的事情。由於是對原表進行克隆，且數據存儲在不同的表空間，因此毫不猶豫地使用了CREATE TABLE AS，結果在運行package時，error...
--1、非空約束遺失
-->使用create table as 來創建對象
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)
-->從上面的desc可以看出新創建的表少了非空約束
-->下面手動爲其增加非空約束，增加後與原來的表是一致的。當然使用create table as時，索引是需要單獨重建的。
scott@CNMMBO> alter table tb_dept modify (deptno not null);
Table altered.
scott@CNMMBO> drop table tb_dept; -->刪除剛剛穿件的表tb_dept
Table dropped.
--2、存在非空約束時default約束遺失
-->下面爲表dept的loc列添加非空約束，且賦予default值
scott@CNMMBO> alter table dept modify (loc default 'BeiJing' not null);
Table altered.
-->爲原始表新增一條記錄
scott@CNMMBO> insert into dept(deptno,dname) select 50,'DEV' from dual;
1 row created.
scott@CNMMBO> commit;
Commit complete.
-->下面的查詢可以看到新增記錄50的loc爲缺省值'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來創建對象
scott@CNMMBO> create table tb_dept as select * from dept;
Table created.
-->從下面可知，由於列loc存在default值，所以此時not null約束被同時賦予
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
-->爲新創建的表新增記錄
-->新增時發現儘管not null約束生效，但原表上設定的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、唯一約束遺失
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
-->有關check約束與外鍵約束不再演示
--4、最徹底的解決辦法
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、演示環境
scott@CNMMBO> select * from v$version where rownum<2;
BANNER
----------------------------------------------------------------
Oracle Database 10g Release 10.2.0.3.0 - 64bit Production
--6、演示結論
-->create table as 儘管會克隆表及數據，數據是完整的，但是結構部分僅僅是部分克隆
-->create table as 會使用表上的約束被遺失或出於非正常狀態
-->create table as 時，表上的索引、觸發器等不會被同時克隆
-->create table as 僅作測試使用，要得到完整的結構語句，還是使用dbms_metadata.get_ddl包