实验室写程序发现的一些SQL技巧总结一下：
1．select count(*) as Num,supplier_city from supplier
where supplier_city is not null group by supplier_city order by Num desc
按城市数目多少统计
2．select company_street from company where len(company_street)>45 or len(company_street)<6
select company_city_name from company
where len(company_city_name)>24 or len(company_city_name)<5
根据长度判断城市和街道是否有效
3．删除重复的记录
select distinct NAME ,HOME into #Tmp from Test
drop table Test
select NAME ,HOME into Test from #Tmp
drop table #Tmp
4．联合查询
delete from company where company_id in
(select company_id from exhibition_company_relation where exhibition_id = '135331')
