Re: Is this possible w/o a stored procedure?

> I'm using mySQL, so stored procedures are not possible.
> I have a simple table:
 

> ID int
> PARENT_ID int
> DESC varchar(10)
 

> So, a top level category would just have an ID and no PARENT_ID. There
> is no limit on how deep the recursion can go.
 

> I want to have a query which will get all IDs under a certain PARENT_ID.

This question comes up every month or so. The only way to handle it really is to use Joe Celko's nested set approach, as described in his "SQL for Smarties" (Morgan-Kaufmann). Below is one of his standard replies to this (I believe there is a typo in one of the queries; left to the reader as an exercise :-). Cheers,

                                                                      Philip

    From: joe_celko_at_my-deja.com
    Subject: Re: tree structure in database     Date: Fri, 13 Aug 1999 01:00:59 GMT               

    Another way of representing trees is to show them as nested sets.     Since SQL is a set oriented language, this is a better model than the     usual adjacency list approach you see in most text books. Let us     define a simple Personnel table like this, ignoring the left (lft) and     right (rgt) columns for now. This problem is always given with a     column for the employee and one for his boss in the textbooks:     

     CREATE TABLE Personnel
     (emp CHAR(10) PRIMARY KEY,
      boss CHAR(10),       -- unneeded & denormalizes the table
      salary DECIMAL(6,2) NOT NULL,
      lft INTEGER NOT NULL,
      rgt INTEGER NOT NULL);
    
     Personnel
     emp      boss      salary  lft  rgt
     ===================================
     Albert   NULL     1000.00   1   12
     Bert     Albert    900.00   2    3
     Chuck    Albert    900.00   4   11
     Donna    Chuck     800.00   5    6
     Eddie    Chuck     700.00   7    8
     Fred     Chuck     600.00   9   10
    
     which would look like this as a directed graph:
    
                Albert (1,12)
                /        \
              /            \
        Bert (2,3)    Chuck (4,11)
                       /    |   \
                     /      |     \
                   /        |       \
                 /          |         \
            Donna (5,6)  Eddie (7,8)  Fred (9,10)
    

    This (without the lft and rgt columns) is called the adjacency list     model, after the graph theory technique of the same name; the pairs of     nodes are adjacent to each other. The problem with the adjacency list     model is that the boss and employee columns are the same kind of thing     (i.e. names of personnel), and therefore should be shown in only one     column in a normalized table. To prove that this is not normalized,     assume that "Chuck" changes his name to "Charles"; you have to change     his name in both columns and several places. The defining     characteristic of a normalized table is that you have one fact, one     place, one time.     

    To show a tree as nested sets, replace the nodes with ovals, then nest     subordinate ovals inside each other. The root will be the largest oval     and will contain every other node. The leaf nodes will be the     innermost ovals with nothing else inside them and the nesting will show     the hierarchical relationship. The rgt and lft columns (I cannot use     the reserved words LEFT and RIGHT in SQL) are what shows the nesting.     

    If that mental model does not work, then imagine a little worm crawling     anti-clockwise along the tree. Every time he gets to the left or right     side of a node, he numbers it. The worm stops when he gets all the way     around the tree and back to the top.     

    This is a natural way to model a parts explosion, since a final     assembly is made of physically nested assemblies that final break down     into separate parts.     

    At this point, the boss column is both redundant and denormalized, so     it can be dropped. Also, note that the tree structure can be kept in     one table and all the information about a node can be put in a second     table and they can be joined on employee number for queries.     

    To convert the graph into a nested sets model think of a little worm     crawling along the tree. The worm starts at the top, the root, makes a     complete trip around the tree. When he comes to a node, he puts a     number in the cell on the side that he is visiting and increments his     counter. Each node will get two numbers, one of the right side and one     for the left. Computer Science majors will recognize this as a odified     preorder tree traversal algorithm. Finally, drop the unneeded     ersonnel.boss column which used to represent the edges of a graph.     

    This has some predictable results that we can use for building     queries. The root is always (left = 1, right = 2 * (SELECT COUNT(*)     FROM TreeTable)); leaf nodes always have (left + 1 = right); subtrees     are defined by the BETWEEN predicate; etc. Here are two common queries     which can be used to build others:     

1. An employee and all their Supervisors, no matter how deep the tree.

     SELECT P2.*
       FROM Personnel AS P1, Personnel AS P2
      WHERE P1.lft BETWEEN P2.lft AND P2.rgt
        AND P1.emp = :myemployee;
    

    2. The employee and all subordinates. There is a nice symmetry here.     

     SELECT P2.*
       FROM Personnel AS P1, Personnel AS P2
      WHERE P1.lft BETWEEN P2.lft AND P2.rgt
        AND P2.emp = :myemployee;
    

    3. Add a GROUP BY and aggregate functions to these basic queries and     you have hierarchical reports. For example, the total salaries which     each employee controls:     

     SELECT P2.emp, SUM(P1.salary)
       FROM Personnel AS P1, Personnel AS P2
      WHERE P1.lft BETWEEN P2.lft AND P2.rgt
      GROUP BY P2.emp;
    

    This will be two to three orders of magnitude faster than the adjacency     list model.     

    For details, see the chapter in my book JOE CELKO'S SQL FOR SMARTIES by     Joe Celko, Morgan-Kaufmann, 1995, ISBN 1-55860-323-9 or my columns in     DBMS magazine in 1996 March, April, May and June issues. I went into     details as to how to manipulate this model.     

    --CELKO--               Sent via Deja.com http://www.deja.com/     Share what you know. Learn what you don't.

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