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 id
• parent_id: the parent category id
• name: 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 the categories 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 the categories table with the tree table on the id and parent_id columns.
• SELECT * FROM tree: This statement selects all the columns from the tree table.

## Helpful links

• PostgreSQL Documentation: WITH
• PostgreSQL Documentation: Recursive Queries

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