HIERARCHICAL QUERIES AND SYS_CONNECT_BY_PATH

Hierarchical Queries

If a table contains hierarchical data, then you can select rows in a hierarchical order using the hierarchical query clause:

hierarchical_query_clause::=

Description of the illustration hierarchical_query_clause.gif

START WITH specifies the root row(s) of the hierarchy.

CONNECT BY specifies the relationship between parent rows and child rows of the hierarchy.

• The NOCYCLE parameter instructs Oracle Database to return rows from a query even if a CONNECT BY LOOP exists in the data. Use this parameter along with the CONNECT_BY_ISCYCLE pseudocolumn to see which rows contain the loop. Please refer to CONNECT_BY_ISCYCLE Pseudocolumn for more information.

• In a hierarchical query, one expression in condition must be qualified with the PRIOR operator to refer to the parent row. For example,

  ... PRIOR expr = expr
  or
  ... expr = PRIOR expr
  
  

  If the CONNECT BY condition is compound, then only one condition requires the PRIOR operator, although you can have multiple PRIOR conditions. For example:

  CONNECT BY last_name != 'King' AND PRIOR employee_id = manager_id ...
  CONNECT BY PRIOR employee_id = manager_id and 
             PRIOR account_mgr_id = customer_id ...
  
  

  PRIOR is a unary operator and has the same precedence as the unary + and - arithmetic operators. It evaluates the immediately following expression for the parent row of the current row in a hierarchical query.

  PRIOR is most commonly used when comparing column values with the equality operator. (The PRIOR keyword can be on either side of the operator.)PRIOR causes Oracle to use the value of the parent row in the column. Operators other than the equal sign (=) are theoretically possible in CONNECT BYclauses. However, the conditions created by these other operators can result in an infinite loop through the possible combinations. In this case Oracle detects the loop at run time and returns an error.

Both the CONNECT BY condition and the PRIOR expression can take the form of an uncorrelated subquery. However, the PRIOR expression cannot refer to a sequence. That is, CURRVAL and NEXTVAL are not valid PRIOR expressions.

You can further refine a hierarchical query by using the CONNECT_BY_ROOT operator to qualify a column in the select list. This operator extends the functionality of the CONNECT BY [PRIOR] condition of hierarchical queries by returning not only the immediate parent row but all ancestor rows in the hierarchy.

SYS_CONNECT_BY_PATH

Syntax

Description of the illustration sys_connect_by_path.gif

Purpose

SYS_CONNECT_BY_PATH is valid only in hierarchical queries. It returns the path of a column value from root to node, with column values separated by char for each row returned by CONNECT BY condition.

Both column and char can be any of the datatypes CHAR, VARCHAR2, NCHAR, or NVARCHAR2. The string returned is of VARCHAR2 datatype and is in the same character set as column.

See Also:

"Hierarchical Queries" for more information about hierarchical queries and  CONNECT  BY conditions

Examples

The following example returns the path of employee names from employee Kochhar to all employees of Kochhar (and their employees):

SELECT LPAD(' ', 2*level-1)||SYS_CONNECT_BY_PATH(last_name, '/') "Path"
   FROM employees
   START WITH last_name = 'Kochhar'
   CONNECT BY PRIOR employee_id = manager_id;

Path
---------------------------------------------------------------
 /Kochhar
   /Kochhar/Greenberg
     /Kochhar/Greenberg/Faviet
     /Kochhar/Greenberg/Chen
     /Kochhar/Greenberg/Sciarra
     /Kochhar/Greenberg/Urman
     /Kochhar/Greenberg/Popp
   /Kochhar/Whalen
   /Kochhar/Mavris
   /Kochhar/Baer
   /Kochhar/Higgins
     /Kochhar/Higgins/Gietz

实验：

SELECT T.EMPNO, ENAME, SUBSTR(SYS_CONNECT_BY_PATH(T.ENAME, ','), 2), T.MGR
  FROM EMP T
 START WITH T.MGR IS NULL
CONNECT BY PRIOR EMPNO = MGR;



SELECT LPAD(' ', LEVEL * 2 - 1) || ENAME AS TREE, F.EMPNO,ENAME,JOB,MGR, LEVEL
  FROM EMP F
 START WITH MGR IS NULL
CONNECT BY PRIOR EMPNO = MGR;