Question

I am not totally sure I am naming this right, but please bear with me.

I am wondering if is possible to do something like this in SQL(MySQL specifically):
Let’s say we have tree-like data that is persisted in the database in the following table:

  mysql> desc data_table;
  +------------------------+---------------------+------+-----+---------+----------------+
  | Field                  | Type                | Null | Key | Default | Extra          |
  +------------------------+---------------------+------+-----+---------+----------------+
  | id                     | int(10) unsigned    | NO   | PRI | NULL    | auto_increment |
  | parent_id              | int(10) unsigned    | YES  | MUL | NULL    |                |
  | value                  | text                | YES  |     | NULL    |                |

So each row has a parent, except for the ‘root’ row and each row has children except for leaf rows.

Is it possible to find all descendants of any given row utilizing solely SQL?

Answer 1

It’s possible to fetch all descendants utilizing solely SQL, but not in a single query. But I’m sure you figured that out; I assume you mean you want to do it in a single query.

You might be interested in reading about some alternative designs to store tree structures, that do enable you to fetch all descendants using a single SQL query. See my presentation Models for Hierarchical Data with SQL and PHP.

You can also use recursive SQL queries with other brands of database (e.g. PostgreSQL), but MySQL does not currently support this feature.