java0day01的博客

--以下两条语句查询结果一样select * from t_gasbrand t1 left join t_userfiles t2 on t1.id = t2.f_gasbrand_id where t1.id = 17 --注意这里必须是where不能是and!!!where是取交集select * from t_gasbrand t1,t_userfiles t2 where t1.id = t2.f_gasbrand_id and t1.id = 17 --如果这样写就错误了s

--以下两条sql语句,查询结果一模一样,区别是,一个是分组前用where加限定条件,一个是分组后,使用having 加限定条件,查询结果都一样.select count(*) '停用总数' from v3.dbo.t_userinfo where f_user_state = '停用' group by f_user_state --group by 分组前,使用where加限定条件select count(*) '停用总数' from v3.dbo.t_userinfo group by f_u