How to select all child of a tree structure in MySQL

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SQL

MySQL

i have table with following structure for a tree:

Int => Id [Primary key]
varchar => Name
Int => ParantId [RefKey:-Ref to Id]

now i want to do is: by selecting any name, i need all the childs and sub child under the name that i selected and not from other names which are parallel or above the selected name, i wrote following query:

SELECT GROUP_CONCAT(o2.Id SEPARATOR ',')
    FROM organization AS o1, organization AS o2
    WHERE o1.Id = o2.ParentOrganizationId
        AND (o1.Id=3 OR o1.ParentOrganizationId =3));

but it is not working!! can any one help me?

thanks in advance:-)