The document discusses querying hierarchical data using Common Table Expressions (CTEs) in SQL, highlighting their availability in MariaDB since version 10.2. It covers both non-recursive and recursive CTEs, including syntax examples, use cases for improved readability, multiple references, and year-over-year comparisons. It also addresses the challenges of handling hierarchical data in SQL and explains how recursive CTEs can efficiently manage such structures.

1. Querying Hierarchical
Data with CTE
ALEXANDER ALDEV
DBaaS Architect
MariaDB Corporation

2. Common Table Expressions
● SQL:1999 standard
● Temporary named result set
● Supported by: Oracle, MS SQL, PostgreSQL, SQLLite, MySQL, …
● Available in MariaDB since 10.2
● Two types:
○ Non-recursive
○ Recursive

3. CTE Syntax
WITH engineers AS (
SELECT *
FROM employees
WHERE dept = ‘engineering’
)
SELECT * FROM engineers ...
CTE nameWITH keyword
CTE body
use in query

4. Similar to Derived Queries
WITH engineers AS (
SELECT *
FROM employees
WHERE dept =
‘engineering’
)
SELECT * FROM engineers ...
SELECT * FROM (
SELECT *
FROM employees
WHERE dept =
‘engineering’
) engineers
...

5. Use Case: Readability
WITH engineers AS (
SELECT *
FROM employees
WHERE dept = ‘engineering’
),
WITH eu_engineers AS AS (
SELECT * FROM engineers
WHERE country IN (‘BG’,’FI’,’DE’)
)
SELECT COUNT(*) FROM eu_engineers

6. Use Case: Multiple References to CTE
WITH engineers AS (
SELECT *
FROM employees
WHERE dept = ‘engineering’
)
SELECT * FROM engineers e1
WHERE NOT EXISTS( SELECT 1 FROM engineers e2
WHERE e1.country = e2.country
AND e1.first = e2.first )

7. Use Case: Year-over-Year comparison
WITH annual_sales AS (
SELECT product, YEAR(invoice_date) AS year,
SUM( amount ) AS total
FROM invoices
GROUP BY 1, 2
)
SELECT y1.year, y1.product, y1.total, y2.total last_yr_total
FROM annual_sales y1
JOIN annual_sales y2
ON y1.year = y2.year + 1
AND y1.product = y2.product

8. Recursive CTE
● SQL does not handle hierarchical data well
● Trees in format (parent_id, child_id)
● Graphs in format (point 1, point 2)
● Recursive CTEs allow for hierarchical data to be handled
● Also useful for search algorithms where next iteration depends on previous

9. Recursive CTE Syntax
WITH RECURSIVE ancestors AS (
SELECT * FROM family
WHERE name = ‘Alex’
UNION ALL
SELECT f.* FROM family f, ancestors
WHERE f.parent_id = ancestors.id
) SELECT * FROM ancestors;
SELECT * FROM engineers ...
RECURSIVE keyword
CTE anchor
recursive use

10. How CTEs work?
● Base execution strategy is to materialize
● Optimizations possible for non-recursive CTE
● Recursive CTEs use this flow:
1. Evaluate anchor expression
2. Evaluate recursive expression -> new data
3. Append new data to result
4. If new data is not-empty goto 2

11. Example:
Sudoku Solver

12. Example:
ETL Job Scheduler

13. THANK YOU!