一句令人费解的SQL查询语句

Question

create table t tablespace assm
as
select 0 id,a.* from all_objects a
where 1=0;红色部分我不是很理解，那位能详细的讲解一下

Answer 1

create table t tablespace assm  --创建表t在表空间assm上
as
select 0 id,a.* from all_objects a  --第一列名称为id，类型和0相同(number)，且默认值为0
where 1=0; --1=0永远不成立，因此只复制表 all_object 的结构，不复制数据。--最后建出一个表，比all_objects 多一列id，但是表中无数据（where 1=0）
[TEST@orcl] SQL>desc t
 名称                                      是否为空? 类型
 ----------------------------------------- -------- ---------------------
 ID                                                 NUMBER
 OWNER                                     NOT NULL VARCHAR2(30)
 OBJECT_NAME                               NOT NULL VARCHAR2(30)
 SUBOBJECT_NAME                                     VARCHAR2(30)
 OBJECT_ID                                 NOT NULL NUMBER
 DATA_OBJECT_ID                                     NUMBER
 OBJECT_TYPE                                        VARCHAR2(19)
 CREATED                                   NOT NULL DATE
 LAST_DDL_TIME                             NOT NULL DATE
 TIMESTAMP                                          VARCHAR2(19)
 STATUS                                             VARCHAR2(7)
 TEMPORARY                                          VARCHAR2(1)
 GENERATED                                          VARCHAR2(1)
 SECONDARY                                          VARCHAR2(1)