Recursive query is a powerful feature that allows us to query hierarchical data. Let’s consider an example to store product taxonomy for an e-commerce database. Let’s assume we have a table called
categories that contains columns
id, name, level, parent_id, where
level indicates the depth of this cateogry and
parent_id is a reference to another entry in
categories table that is a parent of this entry (self-referencing).
Below is an example data.
|128||Clothing And Accessories||0|
|337||Boys Clothing Accessories||2||89|
|376||Girls Clothing Accessories||2||94|
We have Clothing And Accessories as our top-level category which has no parent i.e
NULL and its
level is 0.
Let’s say we wanted to find out all categories under Clothing and Accessories. Getting direct descendents is easy. We just filter on
SELECT * FROM categories WHERE parent_id = 128
But we want to get everything under the given category. Without recursive query we would have to write this logic in our code but thankfully we can do it using SQL query only.
Let’s look at the query to fetch all children
with recursive cte as ( select id, name, level, parent_id from categories c where c.id = 128 -- initial selector union select c2.id, c2.name, c2.level, c2.parent_id from categories c2 join cte on c2.parent_id = cte.id -- recursive part ) select * from cte;
A recursive query consists of two parts. First part where we select the initial rows(s) and the second part where we select more rows based on the rows from the upper part. Also note that the
with statement has
recursive keyword as well.
In the first part we selected a category with id 128 i.e. our Clothing and Accessories category. For sake of explanation, let’s “assign” this result to a temporary table called
Now in the second part, select rows form categories table by joining with
cte where categories in
cte table is parent of these categories. This filter is given by the join condition
c2.parent_id = cte.id. This join will match the categories
[Boys, Girls, Men, Women]. Again for simplicity, let’s assume all these 4 categories are accumulated in
Upto now we have our “main” category as well as direct children of this category. Since this is a recursive query, it is not going to end just now. It will again join the
categories table with
cte. This time, the query is basically saying give me categories which are children of categories in
cte table i.e. children of
[Boys, Girls, Men, Women]. This process is repeated until no results are returned.
Writing this query in SQLAlchemy is also pretty straightforward. We first define a CTE with
recursive=True for the top portion of the query. We then define the bottom part of the query by joining it with the top part and finally applying a
topq = sess.query(Category) topq = topq.filter(Category.id == 128) topq = topq.cte('cte', recursive=True) bottomq = sess.query(Category) bottomq = bottomq.join(topq, Category.parent_id == topq.c.id) recursive_q = topq.union(bottomq) q = sess.query(recursive_q)
I hope this post helped you in understanding recursive query. Please share this if you enjoyed it.