SQL基础-->层次化查询(START BY ... CONNECT BY PRIOR)
层次化查询,即树型结构查询,是SQL中经常用到的功能之一,通常由根节点,父节点,子节点,叶节点组成,其语法如下:
SELECT [LEVEL] ,column,expression,... FROM table_name [WHERE where_clause] [[START WITH start_condition] [CONNECT BY PRIOR prior_condition]]; LEVEL:为伪列,用于表示树的层次 start_condition：层次化查询的起始条件
--使用start with ...connect by prior 从根节点开始遍历
select empno,mgr,ename,job from emp start with empno = 7839 connect by prior empno = mgr;
树型结构遍历过程(通过上面的查询来描述) 1).从根节点开始(即where_clause中的条件,如果为非根节点则分根节点作为根节点开始遍历,如上例empno = 7839) 2).遍历根节点(得到empno = 7839记录的相关信息) 3).判断该节点是否存在由子节点，如果则访问最左侧未被访问的子节点,转到),否则下一步 如上例中prior_condition为empno = mgr,即子节点的mgr等于父节点的empno,在此时mgr为7839的记录 4).当节点为叶节点，则访问完毕,否则,转到) 5).返回到该节点的父节点,转到) --伪列level的使用 --注意connect by prior empno = mgr 的理解 --prior表示前一条记录,即下一条返回记录的mgr应当等于前一条记录的empno
select level,empno,mgr,ename,job from emp start with ename = ‘KING‘ connect by prior empno = mgr order by level;
--获得层次数
select count(distinct level) "Level" from emp start with ename = ‘KING‘ connect by prior empno = mgr;
--格式化层次查询结果(使用左填充* level - 1个空格)
col Ename for a30 select level, lpad(‘ ‘,2 * level - 1) || ename as "Ename", job from emp start with ename = ‘KING‘ connect by prior empno = mgr;
--从非根节点开始遍历(只需修改start with 中的条件即可)
select level, lpad(‘ ‘,2 * level - 1) || ename as "Ename", job from emp start with ename = ‘SCOTT‘ connect by prior empno = mgr;
--从下向上遍历(交换connect by prior中的条件即可,使用mgr = empno) --注意connect by prior mgr = empno 的理解 --prior表示前一条记录,即下一条返回记录的empno应当等于前一条记录的mgr
select level, lpad(‘ ‘,2 * level - 1) || ename as "Ename", job from emp start with ename = ‘SCOTT‘ connect by prior mgr = empno;
--从下向上遍历(也可以将prior置于等号右边,得到相同的结果)
select level, lpad(‘ ‘,2 * level - 1) || ename as "Ename", job from emp start with ename = ‘SCOTT‘ connect by empno = prior mgr;
--从层次查询中删除节点和分支
select level, lpad(‘ ‘,2 * level - 1) || ename as "Ename" ,job from emp where ename != ‘SCOTT‘ --通过where子句来过滤SCOTT用户，但SCOTT的下属ADAMS并没有过滤掉 start with empno = 7839 connect by prior empno = mgr;
--通过将过滤条件由where 子句的内容移动到connect by prior 子句中过滤掉SCOTT及其下属
select level, lpad(‘ ‘,2 * level - 1) || ename as "Ename" ,job from emp start with empno = 7839 connect by prior empno = mgr and ename != ‘SCOTT‘;
--在层次化查询中增加过滤条件或使用子查询
select level, lpad(‘ ‘,2 * level - 1) || ename as "Ename" ,job from emp where sal > 2500 start with empno = 7839 connect by prior empno = mgr;
select level, lpad(‘ ‘,2 * level - 1) || ename as "Ename" ,job from emp where sal > (select avg(sal) from emp) start with empno = 7839 connect by prior empno = mgr ;
