实际生活有很多树形结构的数据,比如公司分为多个部门,部门下分为多个组,组下分为多个员工;省市县的归属;页面菜单栏等等。
如果想查询某个节点的父节点或者子节点,一般通过表自身连接完成,但如果该节点的子节点还有多层结构,就需要使用递归调用。但如果数据量特别大,递归的次数指数级上升,而且查询数据库的次数也指数级上升,导致程序和数据库压力剧增,查询时间特别长。那数据库有没有递归查询语句呢?答案是肯定的。
start with connect by prior 递归查询
create table area_test( id number(10) not null, parent_id number(10), name varchar2(255) not null ); alter table area_test add (constraint district_pk primary key (id)); insert into area_test (ID, PARENT_ID, NAME) values (1, null, "中国"); insert into area_test (ID, PARENT_ID, NAME) values (11, 1, "河南省"); insert into area_test (ID, PARENT_ID, NAME) values (12, 1, "北京市"); insert into area_test (ID, PARENT_ID, NAME) values (111, 11, "郑州市"); insert into area_test (ID, PARENT_ID, NAME) values (112, 11, "平顶山市"); insert into area_test (ID, PARENT_ID, NAME) values (113, 11, "洛阳市"); insert into area_test (ID, PARENT_ID, NAME) values (114, 11, "新乡市"); insert into area_test (ID, PARENT_ID, NAME) values (115, 11, "南阳市"); insert into area_test (ID, PARENT_ID, NAME) values (121, 12, "朝阳区"); insert into area_test (ID, PARENT_ID, NAME) values (122, 12, "昌平区"); insert into area_test (ID, PARENT_ID, NAME) values (1111, 111, "二七区"); insert into area_test (ID, PARENT_ID, NAME) values (1112, 111, "中原区"); insert into area_test (ID, PARENT_ID, NAME) values (1113, 111, "新郑市"); insert into area_test (ID, PARENT_ID, NAME) values (1114, 111, "经开区"); insert into area_test (ID, PARENT_ID, NAME) values (1115, 111, "金水区"); insert into area_test (ID, PARENT_ID, NAME) values (1121, 112, "湛河区"); insert into area_test (ID, PARENT_ID, NAME) values (1122, 112, "舞钢市"); insert into area_test (ID, PARENT_ID, NAME) values (1123, 112, "宝丰市"); insert into area_test (ID, PARENT_ID, NAME) values (11221, 1122, "尚店镇");
常用的select项:
LEVEL:级别
connect_by_root:根节点
sys_connect_by_path:递归路径
select t.*,LEVEL from area_test t start with name ="郑州市" connect by prior id=parent_id
其实,如果单层结构,使用表自身连接也可以实现:
select * from area_test t1,area_test t2 where t1.PARENT_ID = t2.ID and t2.name="郑州市";
当查询节点下有多层数据:
select t.*,LEVEL from area_test t start with name ="河南省" connect by prior id=parent_id
select * from area_test t1,area_test t2 where t1.PARENT_ID = t2.ID and t2.name="河南省";
如果使用自身连接,也只能查到子一级节点的数据,需要遍历子一级节点,递归查询每个子一级节点下的子节点。明显麻烦很多!!!
select t.*,level from area_test t start with name ="郑州市" connect by prior t.parent_id=t.id order by level asc;
select d.*, connect_by_root(d.id) rootid, connect_by_root(d.name) rootname from area_test d where name="二七区" start with d.parent_id IS NULL connect by prior d.id=d.parent_id
select d.*, connect_by_root(d.id) rootid, connect_by_root(d.name) rootname from area_test d start with d.parent_id IS NULL connect by prior d.id=d.parent_id
select id, parent_id, name, sys_connect_by_path(name, "->") namepath, level from area_test start with name = "平顶山市" connect by prior id = parent_id
with tmp(id, parent_id, name) as ( select id, parent_id, name from area_test where name = "平顶山市" union all select d.id, d.parent_id, d.name from tmp, area_test d where tmp.id = d.parent_id ) select * from tmp;
with tmp(id, parent_id, name) as ( select id, parent_id, name from area_test where name = "二七区" union all select d.id, d.parent_id, d.name from tmp, area_test d where tmp.parent_id = d.id ) select * from tmp;
参考文章:MySQL 递归查找树形结构,这个方法太实用了
参考文章:Oracle递归查询
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