postgresqlHow can I create a hierarchical query in PostgreSQL?
A hierarchical query in PostgreSQL can be created using the WITH RECURSIVE
statement. This statement allows for a recursive query that can traverse a tree-like structure. An example of a hierarchical query is shown below:
WITH RECURSIVE tree AS (
SELECT id, parent_id, name
FROM categories
WHERE parent_id IS NULL
UNION
SELECT c.id, c.parent_id, c.name
FROM categories c
JOIN tree ON tree.id = c.parent_id
)
SELECT * FROM tree;
This query will output a table with the following columns:
id
: the category idparent_id
: the parent category idname
: the category name
The output of this query will be a list of categories and their parent categories, forming a hierarchy.
Code explanation
**
WITH RECURSIVE tree AS (
: This statement indicates that a recursive query is being used.SELECT id, parent_id, name
: This statement selects the columns from thecategories
table that will be used in the query.WHERE parent_id IS NULL
: This statement specifies that only the top-level categories should be selected.UNION
: This statement combines the results of two separate queries into one result set.JOIN tree ON tree.id = c.parent_id
: This statement joins thecategories
table with thetree
table on theid
andparent_id
columns.SELECT * FROM tree
: This statement selects all the columns from thetree
table.
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