层次查询是一种确定数据行间关系的一种操作手段。层次查询遍历的是一个树形结构。基本语法如下，以下语法嵌入到标准SQL中即可达到层次查询的目的：
| level,... ...【注释：伪列，用于select子句中，根据数据所处的层次结构自动层次编号】 connect by [nocycle] prior 连接条件 【注释：指定数据之间的连接，其中nocycle需要结合connect_by_iscycle伪列确定出父子节点循环关系，不指定prior则仅显示第一层数据】 [start with 开始条件] 【注释：根节点开始条件】 |
一、测试数据
测试表依旧采用Oracle经典的scott模式下的emp表，结构如下：
EMP:
| create table EMP ( EMPNO NUMBER(4) not null, ENAME VARCHAR2(10), JOB VARCHAR2(9), MGR NUMBER(4), HIREDATE DATE, SAL NUMBER(7,2), COMM NUMBER(7,2), DEPTNO NUMBER(2), SEX VARCHAR2(2) default '男' ) -- Create/Recreate primary, unique and foreign key constraints alter table EMP add constraint PK_EMP primary key (EMPNO) using index; alter table EMP add constraint FK_DEPTNO foreign key (DEPTNO) references DEPT (DEPTNO); |
| SQL> select * from emp; EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO SEX ----- ---------- --------- ----- ----------- --------- --------- ------ --- 7369 SMITH CLERK 7902 1980/12/17 800.00 20 男 7499 ALLEN SALESMAN 7698 1981/2/20 1600.00 300.00 30 女 7521 WARD SALESMAN 7698 1981/2/22 1250.00 500.00 30 女 7566 JONES MANAGER 7839 1981/4/2 2975.00 20 女 7654 MARTIN SALESMAN 7698 1981/9/28 1250.00 1400.00 30 女 7698 BLAKE MANAGER 7839 1981/5/1 2850.00 30 女 7782 CLARK MANAGER 7839 1981/6/9 2450.00 10 女 7788 SCOTT ANALYST 7566 1987/4/19 3000.00 20 男 7839 KING PRESIDENT 1981/11/17 5000.00 10 女 7844 TURNER SALESMAN 7698 1981/9/8 1500.00 0.00 30 女 7876 ADAMS CLERK 7788 1987/5/23 1100.00 20 男 7900 JAMES CLERK 7698 1981/12/3 950.00 30 女 7902 FORD ANALYST 7566 1981/12/3 3000.00 20 女 7934 MILLER CLERK 7782 1982/1/23 1300.00 10 男 14 rows selected |
二、层次查询示例
例：查询员工及其员工领导关系：
| SQL> select empno, level, lpad('|-', level * 2, ' ') || ename as empname, mgr 2 from emp 3 connect by prior empno = mgr 4 start with mgr is null; EMPNO LEVEL EMPNAME MGR ----- ---------- ------------------------------ ----- 7839 1 |-KING 7566 2 |-JONES 7839 7788 3 |-SCOTT 7566 7876 4 |-ADAMS 7788 7902 3 |-FORD 7566 7369 4 |-SMITH 7902 7698 2 |-BLAKE 7839 7499 3 |-ALLEN 7698 7521 3 |-WARD 7698 7654 3 |-MARTIN 7698 7844 3 |-TURNER 7698 7900 3 |-JAMES 7698 7782 2 |-CLARK 7839 7934 3 |-MILLER 7782 14 rows selected |
1、connect_by_isleaf
在一个树形结构中，节点分为根节点和叶子节点两种，用户可以通过connect_by_isleaf伪列判断一个节点是根节点还是叶子节点，如果返回值为0，则表示根节点，返回1则表示叶子节点。
| SQL> select empno, 2 level, 3 lpad('|-', level * 2, ' ') || ename as empname, 4 mgr, 5 decode(connect_by_isleaf, 0, '根节点', 1, ' 叶子节点') as isleaf 6 from emp 7 connect by prior empno = mgr 8 start with mgr is null; EMPNO LEVEL EMPNAME MGR ISLEAF ----- ---------- ------------------------------ ----- ------------ 7839 1 |-KING 根节点 7566 2 |-JONES 7839 根节点 7788 3 |-SCOTT 7566 根节点 7876 4 |-ADAMS 7788 叶子节点 7902 3 |-FORD 7566 根节点 7369 4 |-SMITH 7902 叶子节点 7698 2 |-BLAKE 7839 根节点 7499 3 |-ALLEN 7698 叶子节点 7521 3 |-WARD 7698 叶子节点 7654 3 |-MARTIN 7698 叶子节点 7844 3 |-TURNER 7698 叶子节点 7900 3 |-JAMES 7698 叶子节点 7782 2 |-CLARK 7839 根节点 7934 3 |-MILLER 7782 叶子节点 14 rows selected |
2、connect_by_root
取得某一字段在本层次中根节点的数据名称。
使用方法：connect_by_root 字段名称 [as 别名].
| SQL> select empno, 2 level, 3 lpad('|-', level * 2, ' ') || ename as empname, 4 mgr, 5 decode(connect_by_isleaf, 0, '根节点', 1, ' 叶子节点') as isleaf, 6 connect_by_root ename as rootname 7 from emp 8 connect by prior empno = mgr 9 start with mgr is null; EMPNO LEVEL EMPNAME MGR ISLEAF ROOTNAME ----- ---------- ------------------------------ ----- ------------ ---------- 7839 1 |-KING 根节点 KING 7566 2 |-JONES 7839 根节点 KING 7788 3 |-SCOTT 7566 根节点 KING 7876 4 |-ADAMS 7788 叶子节点 KING 7902 3 |-FORD 7566 根节点 KING 7369 4 |-SMITH 7902 叶子节点 KING 7698 2 |-BLAKE 7839 根节点 KING 7499 3 |-ALLEN 7698 叶子节点 KING 7521 3 |-WARD 7698 叶子节点 KING 7654 3 |-MARTIN 7698 叶子节点 KING 7844 3 |-TURNER 7698 叶子节点 KING 7900 3 |-JAMES 7698 叶子节点 KING 7782 2 |-CLARK 7839 根节点 KING 7934 3 |-MILLER 7782 叶子节点 KING 14 rows selected |
3、sys_connect_by_path(column,char)
该函数根据层次节点关系，自动地将当前根节点中所有相关路径进行显示。
| SQL> select empno, 2 level, 3 lpad('|=', level * 2, ' ') || sys_connect_by_path(ename, ' -> ') as empname, 4 mgr, 5 decode(connect_by_isleaf, 0, '根节点', 1, ' 叶子节点') as isleaf, 6 connect_by_root ename as rootname 7 from emp 8 connect by prior empno = mgr 9 start with mgr is null; EMPNO LEVEL EMPNAME MGR ISLEAF ROOTNAME ----- ---------- -------------------------------------------------- ----- ------------ ---------- 7839 1 |= -> KING 根节点 KING 7566 2 |= -> KING -> JONES 7839 根节点 KING 7788 3 |= -> KING -> JONES -> SCOTT 7566 根节点 KING 7876 4 |= -> KING -> JONES -> SCOTT -> ADAMS 7788 叶子节点 KING 7902 3 |= -> KING -> JONES -> FORD 7566 根节点 KING 7369 4 |= -> KING -> JONES -> FORD -> SMITH 7902 叶子节点 KING 7698 2 |= -> KING -> BLAKE 7839 根节点 KING 7499 3 |= -> KING -> BLAKE -> ALLEN 7698 叶子节点 KING 7521 3 |= -> KING -> BLAKE -> WARD 7698 叶子节点 KING 7654 3 |= -> KING -> BLAKE -> MARTIN 7698 叶子节点 KING 7844 3 |= -> KING -> BLAKE -> TURNER 7698 叶子节点 KING 7900 3 |= -> KING -> BLAKE -> JAMES 7698 叶子节点 KING 7782 2 |= -> KING -> CLARK 7839 根节点 KING 7934 3 |= -> KING -> CLARK -> MILLER 7782 叶子节点 KING 14 rows selected |
| SQL> select empno, 2 level, 3 lpad('|=', level * 2, ' ') || sys_connect_by_path(ename, ' -> ') as empname, 4 mgr, 5 decode(connect_by_isleaf, 0, '根节点', 1, ' 叶子节点') as isleaf, 6 connect_by_root ename as rootname 7 from emp 8 connect by prior empno = mgr 9 and empno != 7698 10 start with mgr is null; EMPNO LEVEL EMPNAME MGR ISLEAF ROOTNAME ----- ---------- -------------------------------------------------- ----- ------------ ---------- 7839 1 |= -> KING 根节点 KING 7566 2 |= -> KING -> JONES 7839 根节点 KING 7788 3 |= -> KING -> JONES -> SCOTT 7566 根节点 KING 7876 4 |= -> KING -> JONES -> SCOTT -> ADAMS 7788 叶子节点 KING 7902 3 |= -> KING -> JONES -> FORD 7566 根节点 KING 7369 4 |= -> KING -> JONES -> FORD -> SMITH 7902 叶子节点 KING 7782 2 |= -> KING -> CLARK 7839 根节点 KING 7934 3 |= -> KING -> CLARK -> MILLER 7782 叶子节点 KING 8 rows selected |
4、order siblings by
在层次查询中，如果最终使用了order by子句进行排序，则会破坏最终的层次结构；而如果想保持层次结构同时在每层内进行排序则需要使用order siblings by子句。
| SQL> select empno, 2 level, 3 lpad('|=', level * 2, ' ') || ename as sortname, 4 lpad('|=', level * 2, ' ') || sys_connect_by_path(ename, ' -> ') as empname, 5 mgr, 6 decode(connect_by_isleaf, 0, '根节点', 1, ' 叶子节点') as isleaf, 7 connect_by_root ename as rootname 8 from emp 9 connect by prior empno = mgr 10 start with mgr is null 11 order by ename; EMPNO LEVEL SORTNAME EMPNAME MGR ISLEAF ROOTNAME ----- ---------- -------------------- -------------------------------------------------- ----- ------------ ---------- 7876 4 |=ADAMS |= -> KING -> JONES -> SCOTT -> ADAMS 7788 叶子节点 KING 7499 3 |=ALLEN |= -> KING -> BLAKE -> ALLEN 7698 叶子节点 KING 7698 2 |=BLAKE |= -> KING -> BLAKE 7839 根节点 KING 7782 2 |=CLARK |= -> KING -> CLARK 7839 根节点 KING 7902 3 |=FORD |= -> KING -> JONES -> FORD 7566 根节点 KING 7900 3 |=JAMES |= -> KING -> BLAKE -> JAMES 7698 叶子节点 KING 7566 2 |=JONES |= -> KING -> JONES 7839 根节点 KING 7839 1 |=KING |= -> KING 根节点 KING 7654 3 |=MARTIN |= -> KING -> BLAKE -> MARTIN 7698 叶子节点 KING 7934 3 |=MILLER |= -> KING -> CLARK -> MILLER 7782 叶子节点 KING 7788 3 |=SCOTT |= -> KING -> JONES -> SCOTT 7566 根节点 KING 7369 4 |=SMITH |= -> KING -> JONES -> FORD -> SMITH 7902 叶子节点 KING 7844 3 |=TURNER |= -> KING -> BLAKE -> TURNER 7698 叶子节点 KING 7521 3 |=WARD |= -> KING -> BLAKE -> WARD 7698 叶子节点 KING 14 rows selected |
| SQL> select empno, 2 level, 3 lpad('|=', level * 2, ' ') || ename as sortname, 4 lpad('|=', level * 2, ' ') || sys_connect_by_path(ename, ' -> ') as empname, 5 mgr, 6 decode(connect_by_isleaf, 0, '根节点', 1, ' 叶子节点') as isleaf, 7 connect_by_root ename as rootname 8 from emp 9 connect by prior empno = mgr 10 start with mgr is null 11 order siblings by ename; EMPNO LEVEL SORTNAME EMPNAME MGR ISLEAF ROOTNAME ----- ---------- -------------------- -------------------------------------------------- ----- ------------ ---------- 7839 1 |=KING |= -> KING 根节点 KING 7698 2 |=BLAKE |= -> KING -> BLAKE 7839 根节点 KING 7499 3 |=ALLEN |= -> KING -> BLAKE -> ALLEN 7698 叶子节点 KING 7900 3 |=JAMES |= -> KING -> BLAKE -> JAMES 7698 叶子节点 KING 7654 3 |=MARTIN |= -> KING -> BLAKE -> MARTIN 7698 叶子节点 KING 7844 3 |=TURNER |= -> KING -> BLAKE -> TURNER 7698 叶子节点 KING 7521 3 |=WARD |= -> KING -> BLAKE -> WARD 7698 叶子节点 KING 7782 2 |=CLARK |= -> KING -> CLARK 7839 根节点 KING 7934 3 |=MILLER |= -> KING -> CLARK -> MILLER 7782 叶子节点 KING 7566 2 |=JONES |= -> KING -> JONES 7839 根节点 KING 7902 3 |=FORD |= -> KING -> JONES -> FORD 7566 根节点 KING 7369 4 |=SMITH |= -> KING -> JONES -> FORD -> SMITH 7902 叶子节点 KING 7788 3 |=SCOTT |= -> KING -> JONES -> SCOTT 7566 根节点 KING 7876 4 |=ADAMS |= -> KING -> JONES -> SCOTT -> ADAMS 7788 叶子节点 KING 14 rows selected |
5、connect_by_iscycle
在进行层次查询时，最重要的是根据指定的数据确定数据间的层次关系，但是有时候会出现死循环的情况。在oracle中提供了connect_by_iscycle伪列用于判断是否出现了死循环，如果出现循环，则返回1，否则返回0.同时要判断是否为循环节点，需要nocycle的支持。
| SQL> update emp set mgr = 7698 where empno = 7839; 1 row updated SQL> commit; Commit complete SQL> select empno, 2 level, 3 lpad('|=', level * 2, ' ') || ename as sortname, 4 decode(connect_by_isleaf, 1, '根节点', 0, ' 叶子节点') as isleaf, 5 decode(connect_by_iscycle, 1, '【X】存在循环', 0, '【√】没有循环') as iscycle, 6 mgr 7 from emp 8 connect by nocycle prior empno = mgr 9 start with mgr = 7839 10 order siblings by ename; EMPNO LEVEL SORTNAME ISLEAF ISCYCLE MGR ----- ---------- -------------------- ------------ -------------- ----- 7698 1 |=BLAKE 叶子节点 【√】没有循环 7839 7499 2 |=ALLEN 根节点 【√】没有循环 7698 7900 2 |=JAMES 根节点 【√】没有循环 7698 7839 2 |=KING 叶子节点 【X】存在循环 7698 7782 3 |=CLARK 叶子节点 【√】没有循环 7839 7934 4 |=MILLER 根节点 【√】没有循环 7782 7566 3 |=JONES 叶子节点 【√】没有循环 7839 7902 4 |=FORD 叶子节点 【√】没有循环 7566 7369 5 |=SMITH 根节点 【√】没有循环 7902 7788 4 |=SCOTT 叶子节点 【√】没有循环 7566 7876 5 |=ADAMS 根节点 【√】没有循环 7788 7654 2 |=MARTIN 根节点 【√】没有循环 7698 7844 2 |=TURNER 根节点 【√】没有循环 7698 7521 2 |=WARD 根节点 【√】没有循环 7698 7782 1 |=CLARK 叶子节点 【√】没有循环 7839 7934 2 |=MILLER 根节点 【√】没有循环 7782 7566 1 |=JONES 叶子节点 【√】没有循环 7839 7902 2 |=FORD 叶子节点 【√】没有循环 7566 7369 3 |=SMITH 根节点 【√】没有循环 7902 7788 2 |=SCOTT 叶子节点 【√】没有循环 7566 7876 3 |=ADAMS 根节点 【√】没有循环 7788 21 rows selected SQL> update emp set mgr = null where empno = 7839; 1 row updated SQL> commit; Commit complete |