W3cubDocs

/MariaDB

Non-Recursive Common Table Expressions Overview

Common Table Expressions (CTEs) are a standard SQL feature, and are essentially temporary named result sets. There are two kinds of CTEs: Non-Recursive, which this article covers; and Recursive.

MariaDB starting with 10.2.1

Common table expressions were introduced in MariaDB 10.2.1.

Non-Recursive CTEs

The WITH keyword signifies a CTE. It is given a name, followed by a body (the main query) as follows: cte_syntax

CTEs are similar to derived tables. For example

WITH engineers AS 
   ( SELECT * FROM employees
     WHERE dept = 'Engineering' )

SELECT * FROM engineers
WHERE ...
SELECT * FROM
   ( SELECT * FROM employees
     WHERE dept = 'Engineering' ) AS engineers
WHERE
...

A non-recursive CTE is basically a query-local VIEW. There are several advantages and caveats to them. The syntax is more readable than nested FROM (SELECT ...). A CTE can refer to another and it can be referenced from multiple places.

A CTE referencing Another CTE

Using this format makes for a more readable SQL than a nested FROM(SELECT ...) clause. Below is an example of this:

WITH engineers AS (
SELECT * FROM employees
WHERE dept IN('Development','Support') ),
eu_engineers AS ( SELECT * FROM engineers WHERE country IN('NL',...) )
SELECT
...
FROM eu_engineers;

Multiple Uses of a CTE

This can be an 'anti-self join', for example:

WITH engineers AS (
SELECT * FROM employees
WHERE dept IN('Development','Support') )

SELECT * FROM engineers E1
WHERE NOT EXISTS
   (SELECT 1 FROM engineers E2
    WHERE E2.country=E1.country
    AND E2.name <> E1.name );

Or, for year-over-year comparisons, for example:

WITH sales_product_year AS (
SELECT product, YEAR(ship_date) AS year,
SUM(price) AS total_amt
FROM item_sales
GROUP BY product, year )

SELECT *
FROM sales_product_year CUR,
sales_product_year PREV,
WHERE CUR.product=PREV.product 
AND  CUR.year=PREV.year + 1 
AND CUR.total_amt > PREV.total_amt

Another use is to compare individuals against their group. Below is an example of how this might be executed:

WITH sales_product_year AS (
SELECT product,
YEAR(ship_date) AS year,
SUM(price) AS total_amt
FROM item_sales
GROUP BY product, year
)

SELECT * 
FROM sales_product_year S1
WHERE
total_amt > 
    (SELECT 0.1 * SUM(total_amt)
     FROM sales_product_year S2
     WHERE S2.year = S1.year)
Content reproduced on this site is the property of its respective owners, and this content is not reviewed in advance by MariaDB. The views, information and opinions expressed by this content do not necessarily represent those of MariaDB or any other party.

© 2019 MariaDB
Licensed under the Creative Commons Attribution 3.0 Unported License and the GNU Free Documentation License.
https://mariadb.com/kb/en/non-recursive-common-table-expressions-overview/