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Displaying posts with tag: CTE (reset)
Introduction to MySQL 8.0 Recursive Common Table Expression (Part 2)

This is the second part of a two-articles series. In the first part, we introduced the Common Table Expression (CTE), a new feature available on MySQL 8.0 as well as Percona Server for MySQL 8.0. In this article, we’ll present the Recursive Common Table Expression. SQL is generally poor at recursive structures, but it is now possible on MySQL to write recursive queries. Before MySQL 8.0, recursion was possible only by creating stored routines.

What is a Recursive Common Table Expression?

A recursive CTE is one having a subquery that refers to its own name. It is particularly useful in the following cases:

  • To generate series
  • Hierarchical or tree-structured data traversal
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Introduction to MySQL 8.0 Common Table Expressions (Part 1)

This blog is the first part of a two-articles series. In this article, I’m going to introduce the Common Table Expression (CTE), a new feature available on MySQL 8.0, as well as Percona Server for MySQL 8.

What is a Common Table Expression?

We can define a CTE as an alternative to a derived table. In a small way, CTE simplifies complex joins and subqueries, improving the readability of the queries. CTE is part of ANSI SQL 99 and was introduced in MySQL 8.0.1. The same feature is available even on Percona Server for MySQL 8.0.

The main reasons for using CTE are:

  • Better readability of the queries
  • Can be referenced multiple times in the same query
  • Improved performance
  • A valid alternative to a VIEW, if your user cannot create VIEWs
  • Easier chaining of multiple CTE …
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Fun with Bugs #80 - On MySQL Bug Reports I am Subscribed to, Part XVI

Today I'd like to continue my review of public MySQL bug reports with a list of some bugs I've subscribed to over last 3 weeks. It's already long enough and includes nice cases to check and share. Note that I usually subscribe to a bug either because it directly affects me or customers I work with, or I consider it technically interesting (so I mostly care about InnoDB, replication, partitioning and optimizer bugs), or it's a "metabug" - a problem in the way public bug report is handled by Oracle engineers. These are my interests related to MySQL bugs.

As usual, I start with the oldest bugs and try to mention bug reporters by name with links to their other reports whenever this may give something useful to a reader. I try …

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30 mins with MySQL JSON functions

JSON (JavaScript Object Notation) is a popular way for moving data between various systems, including databases. Starting with 5.7 MySQL implemented a native JSON data type and a set of JSON functions that allows you to perform operations on JSON values.

MySQL 8.0 new features in real life applications: roles and recursive CTEs

I am happy that the MySQL team is, during the last years, blogging about each major feature that MySQL Server is getting; for example, the series on Recursive Common Table Expressions. Being extremely busy myself, …

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Common Table Expressions in MySQL

In a recent labs release a new feature was introduced by Oracle, or actually two very related new features were introduced. The first new feature is Common Table Expressions (CTEs), which is also known as WITH. The second feature is recursive CTEs, also known as WITH RECURSIVE.

An example of WITH:

WITH non_root_users AS (SELECT User, Host FROM mysql.user WHERE User<>'root')
SELECT Host FROM non_root_users WHERE User = ?

The non-CTE equivalent is this:

SELECT Host FROM 
(SELECT User, Host FROM mysql.user WHERE User<>'root') non_root_users
WHERE User = ?

This makes it easier to understand the query, especially if there are many subqueries.

Besides using regular subqueries or CTEs you could also put the subquery in a view, but this requires more …

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MariaDB's RETURNING feature.

There is a new feature in the MariaDB 10 Beta which caught my eye: support for returning a result set on delete.

With a 'regular' DELETE operation you only get to know the number of affected rows. To get more info or actions you have to use a trigger or a foreign key. Anoter posibility is doing a SELECT and then a DELETE and with the correct transaction isolation a transactional support this will work.

With the support for the RETURNING keyword this has become easier to do and it will probably bennefit performance and save you a few roundtrips and a few lines of code.

There is already support for RETURNING in PostgreSQL. And PostgreSQL has an other nifty feature for which RETURNING really helps: CTE or common table expressions or the WITH keyword. I really hope to see CTE support in MySQL or MariaDB some day.

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Showing entries 1 to 7