EXPLAIN EXTENDED

Continuing on the theme of hierarchical queries in MySQL:

• Hierarchical queries in MySQL
• Hierarchical queries in MySQL: adding level
• Hierarchical queries in MySQL: adding ancestry chains.
• Hierarchical queries in MySQL: finding leaves
• Hierarchical queries in MySQL: finding loops
• Hierarchical data in MySQL: parents and children in one query

Assume we have a table with hierarchical structure like this:

We have two trees here: one starting from 1, another one starting from 2.

The problem is: given any item, we should identify the whole tree this item belongs to, and return the whole tree in the hierarchical order.

This also can be easily done using hierarchical queries in MySQL.

In this article: Hierarchical queries in MySQL I shown how to implement a function that returnes tree items in correct order, being called sequentially.

This function is reentrable and keeps its state in session variables, one of which, @start_with, defines the parent element for the tree we want to build.

We have two problems here:

1. Given an item, define a root of the tree it belongs to
2. Build a whole tree, starting from the root

The first problem can be solved by iterating the linked list backwards, starting from the variable given as an input.

This article:

• Sorting lists

describes how to do it in great detail, that's why I'll just put a query here.

Let's create the table described above:

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