CODEWITHSUNDEEP
mysqlsqloracle-database
Adam Arold
Adam Arold

Reputation: 30568

Is there a `connect by` alternative in MySQL?

If I use Oracle there is the connect by keyword which can be used to create hierarchical queries. Currently I'm using MySQL on a project and I would like to know whether there is an alternative for connect by in MySQL?

I tried google but for no avail so far. What I'm trying to achieve is to fetch a tree from the database with one query. There are two tables involved:

areas and area_to_parent_join. The latter contains two ids one is area_id and the other is parent_id. So it is basically a self-join and I can create graphs using that model. In fact it is currently only used to create trees but this might change in the future. But in either case what I would like to have is just a spanning tree.

Edit: areas might have more than 1.000.000 records which makes most of the space-intensive options unfeasible.

Upvotes: 7

Views: 10980

Answers (2)

dvand
dvand

Reputation: 11

In my application, I rarely need to ask for the entire subtree. So, to get away from the big issue of o2, I use 3-deep association / closure table lookups - populating the table with ancestor - descendent only for child, parent, and grandparents. Just recognize that you get what you put in - i.e. don't query for the entire tree without a stored procedure.

Upvotes: 1

STT LCU
STT LCU

Reputation: 4330

As said in comments, there isn't a short way with mysql.

BUT!

If you have the possibility to change the database structure, you can deploy a better design to handle tree-like hierarchies.

If you follow THIS TUTORIAL from Bill Karwin (HERE is the original answer which references that slideshow tutorial), you can find 4 methods used to model an hierarchical structure:

  1. Adiacency List
  2. Path Enumeration
  3. Nested sets
  4. Closure table

Now, the best model possible is the 4th one (I leave descriptions of the other 3 models to the reader), which basically needs 2 tables: one for the elements and one for the paths. In the paths table (the closure table itself), you'll store every path from each node to every descendant (not just the direct childs!).

It's suggested to save also the path length for each row, because it makes easier to query for immediate childrens in the tree.

Even if this solution requires more space, it has the best overall performance and it's really easy to use: it doesn't rely on recursive queries at all AND it will grants referential integrity for the whole dataset!

For example, to get every child of the node #4:

select a.*
from nodes a
join paths b
on a.node_id = b.descendant
where b.ancestor = 4

Another example: get all the ancestors of the node #11

select a.*
from nodes a
join paths b
on a.node_id = b.ancestor
where b.descendant = 11

need to delete the subtree of the node #6

delete from paths where descendant in
(select descendant from paths where ancestor = 6)

Upvotes: 10

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