Question

I have a categories table in MySql something like this:

categoryId | categoryTitle | definedField | parentId
    1              Title         123           NULL
    2          AnotherTitle       234          1
    3         AndAnotherOne       NULL         1

What I need to do is find the closest definedField value by going up to parent,like this;

Since category 2 has a definedField, return its value;
Since category 3 does not have a definedField, search up, to its parent. It has definedField, so return it. If it didn’t have one, search up until find one.

There will ALLWAYS be the topmost category that will have definedField set. I only need to find a good algorithm to search for this in a MySQL InnoDb table.

Answer 1

There is no direct way of retrieving hierarchical data in MySQL (like, for example, Postgres’s RECURSIVE query). There is a good article summarizing different ways of implementing nested data set in MySQL: http://mikehillyer.com/articles/managing-hierarchical-data-in-mysql/

Most users at one time or another have dealt with hierarchical data in
a SQL database and no doubt learned that the management of
hierarchical data is not what a relational database is intended for.
The tables of a relational database are not hierarchical (like XML),
but are simply a flat list. Hierarchical data has a parent-child
relationship that is not naturally represented in a relational
database table.

The article covers two models: Adjacency List and Nested Set.

The Adjacency List Model

In the adjacency list model, each item in the table contains a pointer
to its parent. The topmost element, in this case electronics, has a
NULL value for its parent. The adjacency list model has the advantage
of being quite simple, it is easy to see thatFLASH is a child ofmp3
players, which is a child of portable electronics, which is a child of
electronics. While the adjacency list model can be dealt with fairly
easily in client-side code, working with the model can be more
problematic in pure SQL.

The Nested Set Model

In the Nested Set Model, we can look at our hierarchy in a new way,
not as nodes and lines, but as nested containers.