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Tip of the Day
Language: Enterprise
Expertise: Advanced
Nov 1, 2000

WEBINAR:

On-Demand

Building the Right Environment to Support AI, Machine Learning and Deep Learning


Use Of 'Connect By' clause for Hierarchal Query


The Connect By clause is useful for tables which have 2 or more columns with parent-child relationships. While displaying rows, we can use 'LEVEL' pseudo column to indent the results as hierarchy.

Consider the following example of a community table:
 
Community_Id
Parent_Id
Community_Name

tmr>Select * from Communities;

COMMUNITY_ID  PARENT_ID COMMUNITY_NAME
------------ ---------- -------------------------
           1          0 TV Entertainment
           2          1 NBC
           3          1 ABC
           4          1 FOX
           5          2 Sports
           6          2 Movies
           7          3 Talk Show
           8          3 Documentary
           9          7 Educational
          10          7 Fun

10 rows selected.

/* Find all children, all leaf lets From Root*/
 SELECT LPAD(' ',3*(LEVEL-1)) || substr(Community_Name, 1, 30) Community ,
         Community_id, Parent_Id
 FROM   Communities
 CONNECT BY PRIOR Community_id = Parent_Id
 START WITH Parent_id= 0

Results:

COMMUNITY                           COMMUNITY_ID  PARENT_ID
----------------------------------- ------------ ----------
TV Entertainment                               1          0
   NBC                                         2          1
      Sports                                   5          2
      Movies                                   6          2
   ABC                                         3          1
      Talk Show                                7          3
         Educational                           9          7
         Fun                                  10          7
      Documentary                              8          3
   FOX                                         4          1

10 rows selected.

/* Find immediate children For Parent_Id = 1 : Just go 1 level Down  */
 SELECT LPAD(' ',3*(LEVEL-1)) || substr(Community_Name, 1, 30) Community ,
         Community_id, Parent_Id
 FROM   Communities
 CONNECT BY Community_Id = PRIOR  Parent_Id
 START WITH Parent_Id= 1

Results:

COMMUNITY                           COMMUNITY_ID  PARENT_ID
----------------------------------- ------------ ----------
NBC                                            2          1
   TV Entertainment                            1          0
ABC                                            3          1
   TV Entertainment                            1          0
FOX                                            4          1
   TV Entertainment                            1          0

6 rows selected.


/* Start At Node Community_Id = 7: Traverse up all the way till root */
 SELECT LPAD(' ',3*(LEVEL-1)) || substr(Community_Name, 1, 30) Community ,
         Community_id, Parent_Id
 FROM   Communities
 CONNECT BY Community_id = PRIOR  Parent_Id
 START WITH Community_id = 7

Results:

COMMUNITY                           COMMUNITY_ID  PARENT_ID
----------------------------------- ------------ ----------
Talk Show                                      7          3
   ABC                                         3          1
      TV Entertainment                         1          0


/* Find all children, upto some LEVEL */
 SELECT LPAD(' ',3*(LEVEL-1)) || substr(Community_Name, 1, 30) Community ,
         Community_id, Parent_Id
 FROM   Communities
 Where  LEVEL <= 2
 CONNECT BY PRIOR Community_Id = Parent_Id
 START WITH Parent_Id= 0

Results:
 
COMMUNITY                           COMMUNITY_ID  PARENT_ID
----------------------------------- ------------ ----------
TV Entertainment                               1          0
   NBC                                         2          1
   ABC                                         3          1
   FOX                                         4          1
Trupti Rajparia
 
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