SQL去除某一字段值重复记录的方法
原理：对需要去重复记录的字段按组排序，然后取其中一条记录。在总查询语句中使用in语法过滤
去掉重复记录
select * from company where comid in (select Max(comid) from company group by companyname)
得到重复记录数
select * from company where comid not in (select Max(comid) from company group by companyname)
对完全相同的记录可以使用DISTINCT 对记录进行唯一性过滤
相关文档：
Differring Constraints:
Constraints can have the following attributes: DEFFERRABLE / NOT DEFFERRABLE, INITIALLY DEFFERRED / INITIALLY IMMEDIATE.
e.g.:
alter table dept2 add constraint dept2_id_pk primary key (department_id) deferrable initially deferred; // deferring constraint on creation. ......
CREATE VIEW MYVIEW
AS
SELECT * from bjxxdiweb_database2007.dbo.bm_tongji
UNION ALL
SELECT * from aa.DBO.chen
select * into aa..chen from bjxxdiweb_database2007.dbo.bm_tongji where 1=2
说明：数据库A的表的字段名必须和数据库B的表的字段名相同，包括数据类型等。 ......
PairWise subquery:
e.g.:
select * from wf_docsort where (ndocsortid,nmoduleinfoid) in (select ndocsortid, nmoduleinfoid from wf_docsort where instr(cname,'文')>0)
the above sql is the same function as:
select * from wf_docsort where ndocsortid = (select ndocsortid from wf_docsort where ......
in 的话， 如果是null 就不比较了，既不是in 也不是 not in
exists的话 因为用 = 加在条件里比较了，所以 null 是 not exists
select *
from pricetemp
where cast(商品コード as varchar(10))not in(
select shohin_cd
&nbs ......
