A relational database does not store data in a hierarchical way. Then how do I get the data in a hierarchical manner? Here we get to know about how to use the hierarchical querying feature which Oracle has given. This article talks about how you can interpret the hierarchical query conceptually and build hierarchical queries catering your needs.
Using hierarchical queries, you can retrieve records from a table by their natural relationship. Be it a family tree or a employee/manager tree or what ever.
Tree walking enables you to construct a hierarchical tree if the relationship lie in the same table. For instance, a manager column which exists in the emp table which defines the managerial hierarchy.
We shall take up an example of the emp table in Scott schema. Here King is top most in the hierarchy
empno ename job mgr hiredate 7369 SMITH CLERK 7902 17-Dec-80 7499 ALLEN SALESMAN 7698 20-Feb-81 7521 WARD SALESMAN 7698 22-Feb-81 7566 JONES MANAGER 7839 2-Apr-81 7654 MARTIN SALESMAN 7698 28-Sep-81 7698 BLAKE MANAGER 7839 1-May-81 7782 CLARK MANAGER 7839 9-Jun-81 7788 SCOTT ANALYST 7566 19-Apr-87 7839 KING PRESIDENT 17-Nov-81 7844 TURNER SALESMAN 7698 8-Sep-81 7876 ADAMS CLERK 7788 23-May-87 7900 JAMES CLERK 7698 3-Dec-81 7902 FORD ANALYST 7566 3-Dec-81 7934 MILLER CLERK 7782 23-Jan-82
If we have to query the employees reporting to King directly,
SELECT empno, ename, job, mgr, hiredate FROM emp WHERE mgr = 7839 7566 JONES MANAGER 7839 2-Apr-81 7698 BLAKE MANAGER 7839 1-May-81 7782 CLARK MANAGER 7839 9-Jun-81
But if we have to walk down the tree and check who all are reporting to Jones, Blake and Clark (recursively)
SELECT empno, ename, job, mgr, hiredate FROM emp START WITH mgr IS NULL CONNECT BY PRIOR empno = mgr
We will quickly see what are all the key words used in this query.
START WITH – Specifies the root rows of the hierarchy or in other words, where to start parsing from. This clause is necessary for true hierarchical queries
CONNECT BY PRIOR – This explains the relationship between the parent and the child.
PRIOR – This is used to achieve the recursive condition (The actual walking)
Direction of walking the tree
To explain more on the CONNECT BY clause, this is used to determine if you are walking from top to bottom or bottom to top.
CONNECT BY PRIOR col_1 = col_2
If walking from top to bottom
col_1 is the parent Key(One which identifies the parent) and col_2 is the child key (this identifies the child)
And here it is
CONNECT BY PRIOR empno = mgr SELECT empno, ename, job, mgr, hiredate, level FROM emp START WITH mgr IS NULL CONNECT BY PRIOR empno = mgr
Gets me this result
7839 KING PRESIDENT 17-Nov-81 1 7566 JONES MANAGER 7839 2-Apr-81 2 7788 SCOTT ANALYST 7566 19-Apr-87 3 7876 ADAMS CLERK 7788 23-May-87 4 7902 FORD ANALYST 7566 3-Dec-81 3 7369 SMITH CLERK 7902 17-Dec-80 4 7698 BLAKE MANAGER 7839 1-May-81 2 7499 ALLEN SALESMAN 7698 20-Feb-81 3 7521 WARD SALESMAN 7698 22-Feb-81 3 7654 MARTIN SALESMAN 7698 28-Sep-81 3 7844 TURNER SALESMAN 7698 8-Sep-81 3 7900 JAMES CLERK 7698 3-Dec-81 3 7782 CLARK MANAGER 7839 9-Jun-81 2 7934 MILLER CLERK 7782 23-Jan-82 3
If walking from bottom to top
Col_1 should be the child key and col_2 should be the parent key
CONNECT BY PRIOR mgr = empno
Using Level
LEVEL psedo column shows the level or rank of the particular row in the hierarchical tree. If you see the below query, It shows the level of KING and the level of the guys reporting directly to him
SELECT empno, ename, job, mgr, hiredate, LEVEL FROM emp WHERE LEVEL <= 2 START WITH mgr IS NULL CONNECT BY PRIOR empno = mgr
empno ename job mgr hiredate level 7839 KING PRESIDENT 17-Nov-81 1 7566 JONES MANAGER 7839 2-Apr-81 2 7698 BLAKE MANAGER 7839 1-May-81 2 7782 CLARK MANAGER 7839 9-Jun-81 2
Here The level is used in the where clause to restrict the records till the second level.
Level also can be used to format the Output to form a graph structure
SELECT Lpad(ename,Length(ename) + LEVEL * 10 - 10,'-') FROM emp START WITH mgr IS NULL CONNECT BY PRIOR empno = mgr KING ----------JONES --------------------SCOTT ------------------------------ADAMS --------------------FORD ------------------------------SMITH ----------BLAKE --------------------ALLEN --------------------WARD --------------------MARTIN --------------------TURNER --------------------JAMES ----------CLARK --------------------MILLER
Pruning branches/children
There might be business requirements to partially retrieve a hierarchical tree and to prune branches.
If you do not want to do so, use the where condition to restrict the branch but process the child row
SELECT empno, ename, job, mgr, hiredate FROM emp WHERE ename <> 'JONES' START WITH mgr IS NULL CONNECT BY PRIOR empno = mgr
This will restrict the value Jones in the result set but will still will retrieve Scott and Ford.
Please refer to the attached Picture to get a complete understanding.
To Restrict the value clark and its children, you should be adding the condition after the CONNECT BY
SELECT empno, ename, job, mgr, hiredate FROM emp START WITH mgr IS NULL CONNECT BY PRIOR empno = mgr AND ename <> 'CLARK'
文章轉載自：http://www.orafaq.com/node/2038