Sql 语句

 一，数据查询

 1。基本查询

      select  [ all | distinct ] * from tablename;(distinct :去掉重复行)

      select  t.name as username  from tablename t ;(count(*),avg(字段),sum(字段),min(字段),max(字段))

      select  t.id from tablename t where t.namein ('','');(all,any ,between,exists,in,like,some,is null,is not null)

        order by desc;asc;

       select  sex,count(sex) as 人数 from usertable group by sex;（ having *;必须和group by 连用，作用于组）

   2.嵌套查询

       select * from user t where t.id = (select id from userrole where role = 1) 

      select  t.id ,t.name from user t where   t.depart = 'A部门' and  t.wage > any (select m.wage from user where m.depart = 'B部门')；（all ：最高；any：最低）

     select * from *inner join * on * where *;

       左外连接：

                  select a.id ,a.name ,b.depart from A a  left outer join B bon a.id = b.userid;

                   (左外连接限制b表中的行,而没有显示表a中的行，即a中不符合的数据也显示，但是不符合连接条件的数值都为null)

       右外连接：

                select a.id ,a.name ,b.depart from A a  right outer join B bon a.id = b.userid;

                  (与左连接相反)

      全外连接：

                        full outer join

     交叉连接：cross join