Question

Below is what I have in my table.

myTable

++++++++++++++++++++
Parent   +  Child
++++++++++++++++++++
  C1     +    G1
  C1     +    G2
  C1     +    G3
  G3     +    D1
  G3     +    D2
  C1     +    G4
  G4     +    D3
  G4     +    D4
  C2     +    G5
  C2     +    G6
  C2     +    G7
  C2     +    G8
 ++++++++++++++++++++

What, I want is as below using MYSQL.

C1
  G1
  G2
  G3
    D1
    D2
  G4
    D3
    D4

C2
  G5
  G6
  G7
  G8

Please let me know if this is possible in MYSQL. The output is something like TREE.

Update 1

If I get new table like below is also fine so that I can use this example.

++++++++++++++++++++++++++++++++++++++++
Parent   +  Child   + PLevel  + CLevel
++++++++++++++++++++++++++++++++++++++++
  C1     +    G1    +   1    +   2
  C1     +    G2    +   1    +   2
  C1     +    G3    +   1    +   2
  G3     +    D1    +   2    +   3
  G3     +    D2    +   2    +   3
  C1     +    G4    +   1    +   2
  G4     +    D3    +   2    +   3
  G4     +    D4    +   2    +   3
  C2     +    G5    +   1    +   2
  C2     +    G6    +   1    +   2
  C2     +    G7    +   1    +   2
  C2     +    G8    +   1    +   2
++++++++++++++++++++++++++++++++++++++++

NOTE : I have started level with 1 (in example I have level starting from 0). If I get this new table with level starting from 0 is also fine.

Answer 1

Although you can’t do with a single query, you can do with a stored procedure… The only pre-requirement, you need to add 2 more records to your existing sample table to represent that “C1” and “C2” ARE the top level… Add a record where the “Parent” field is blank, and the child level is “C1” and another for “C2”. This will “prepare” the top-most parent level. for subsequent hierarchy association, otherwise you have no starting “basis” of the top-level hierarchy. It also requires a “primary key” column (which I’ve created in this script as “IDMyTable” which is just 1-x sequential, but would assume you have an auto-increment column on your table to use instead).

I’ve included all the output columns to show HOW it’s built, but the premise of this routine is to create a table based on the expected column outputs, yet extra to hold the hierarchical representation downstream as it’s being built. To MAKE SURE they retain the correct orientation as the layers get deeper, I’m concatinating the “ID” column — you’ll see how it works in the final result set.

Then, in the final result set, I am pre-padding spaces based on however deep the hierarchy data is.

The loop will add any records based on their parent being found in the preceding result set, but only if the ID has not already been added (prevent duplicates)…

To see how the cyclical order was constantly appended to, you can run the last query WITHOUT the order by and see how each iteration qualified and added the previous hierarchy level was applied…

-- --------------------------------------------------------------------------------
-- Routine DDL
-- Note: comments before and after the routine body will not be stored by the server
-- --------------------------------------------------------------------------------
DELIMITER $$

CREATE DEFINER=`root`@`localhost` PROCEDURE `GetHierarchy2`()
BEGIN
    -- prepare a hierarchy level variable 
    set @hierlvl := 00000;

    -- prepare a variable for total rows so we know when no more rows found
    set @lastRowCount := 0;

    -- pre-drop temp table
    drop table if exists MyHierarchy;

    -- now, create it as the first level you want... 
    -- ie: a specific top level of all "no parent" entries
    -- or parameterize the function and ask for a specific "ID".
    -- add extra column as flag for next set of ID's to load into this.
    create table MyHierarchy as
    select 
            t1.IDMyTable,
            t1.Child AS Parent,
            @hierlvl as IDHierLevel,
            cast( t1.IDMyTable as char(100)) FullHierarchy
        from
            MyTable t1
        where
                t1.Parent is null
            OR t1.Parent = '';


    -- how many rows are we starting with at this tier level
    set @lastRowCount := ROW_COUNT();

    -- we need to have a "primary key", otherwise our UPDATE
    -- statement will nag about an unsafe update command
    alter table MyHierarchy add primary key (IDMyTable);


    -- NOW, keep cycling through until we get no more records
    while @lastRowCount > 0 do

        -- NOW, load in all entries found from full-set NOT already processed
        insert into MyHierarchy
            select 
                    t1.IDMyTable,
                    t1.Child as Parent,
                    h1.IDHierLevel +1 as IDHierLevel,
                    concat_ws( ',', h1.FullHierarchy, t1.IDMyTable ) as FullHierarchy
                from
                    MyTable t1
                        join MyHierarchy h1
                            on t1.Parent = h1.Parent
                    left join
                        MyHierarchy h2
                            on t1.IDMyTable = h2.IDMyTable
                where
                    h2.IDMyTable is null;


        set @lastRowCount := row_count();

        -- now, update the hierarchy level
        set @hierLevel := @hierLevel +1;

    end while;


    -- return the final set now
    select 
            *, concat( lpad( ' ', 1 + (IDHierLevel * 3 ), ' ' ), Parent ) as ShowHierarchy
        from MyHierarchy
        order by FullHierarchy;

END