Combining Multiple SELECT Queries in ClickHouse for Efficient Data Retrieval скачать в хорошем качестве

Combining Multiple SELECT Queries in ClickHouse for Efficient Data Retrieval

Learn how to combine multiple `SELECT` queries using `WITH` and `VALUES` in ClickHouse, including alternatives using arrays of tuples. --- This video is based on the question https://stackoverflow.com/q/73531314/ asked by the user 'Mike Pryadko' ( https://stackoverflow.com/u/13334340/ ) and on the answer https://stackoverflow.com/a/73560126/ provided by the user 'Mike Pryadko' ( https://stackoverflow.com/u/13334340/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions. Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Combine few same SELECT queries into one with set of "where"-data Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l... The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license. If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com. --- Efficiently Combining Multiple SELECT Queries in ClickHouse When working with SQL databases, it’s common to encounter situations where you need to retrieve data based on multiple input values. This can often lead to repetitive SELECT queries, which can undermine performance efficiency. If you are using ClickHouse, there’s a way to combine these queries into a more efficient format. The Problem Suppose you have a simple query in ClickHouse that looks like this: [[See Video to Reveal this Text or Code Snippet]] In many cases, however, you may want results for multiple sets of values, such as: [[See Video to Reveal this Text or Code Snippet]] Doing this can make the query lengthy and hard to manage. Additionally, if you need to identify which results come from which data source, it can complicate things further. While this can be easily handled in PostgreSQL with a WITH statement and VALUES, ClickHouse initially seems more restrictive. The Initial Attempts A common workaround that was initially found involves using UNION ALL: [[See Video to Reveal this Text or Code Snippet]] However, this method can become tedious and is not the most efficient way to do it. The Solution Found The breakthrough came when it was discovered that ClickHouse does allow VALUES inside WITH, but without the need for braces, as you would in PostgreSQL. Here’s an example of the more efficient query: [[See Video to Reveal this Text or Code Snippet]] This method simplifies the data retrieval process. However, take note that extra privileges may be required to execute these types of queries: [[See Video to Reveal this Text or Code Snippet]] This means you'll need CREATE TEMPORARY TABLE privileges to run such queries, which may not always be available to every user in a shared database environment. Alternative Approach: Using Arrays of Tuples If obtaining additional privileges is not feasible, there is a fallback method using arrays of tuples. Here's how you can achieve this: [[See Video to Reveal this Text or Code Snippet]] This method allows you to effectively unpack arrays and join them back into your query without the need for additional privileges, thus maintaining a flexible workflow. Final Thoughts Combining multiple SELECT queries in ClickHouse can enhance both the readability and performance of your database interactions. Whether you choose the VALUES method (considering privilege constraints) or the array of tuples approach, you can streamline your data retrieval processes effectively. By keeping these techniques in your SQL toolbox, you'll be better equipped to tackle similar challenges in the future, ultimately making your work with databases more efficient.