where条件查询,条件语句中的子查询，建表查询，建空表查询

1简单的where查询

 select g3e_username from g3e_component where g3e_id >1000;

2子查询（涉及到两个表）（涉及到的查询条件两表格要有相同点）

select * from g3e_component  where g3e_name in (select g3e_name in g3e_feature);

3建空表查询(关键在于多了语句：where 1<>1)as

create table tmp_g3e_component  as select * from g3e_component where 1<>1