Скачать или смотреть How to Get topic_id from Multiple Tables in MySQL

Discover how to effectively retrieve `topic_id` from multiple tables in MySQL, including handling bookmarks and ignored topics in your SQL queries.
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This video is based on the question https://stackoverflow.com/q/66117378/ asked by the user 'Lars Vegas' ( https://stackoverflow.com/u/3990179/ ) and on the answer https://stackoverflow.com/a/66117497/ provided by the user 'ScaisEdge' ( https://stackoverflow.com/u/3522312/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Introduction

In the realm of database management, retrieving relevant data from multiple tables is a fundamental task. Imagine a scenario where you are managing a forum with several tables that store important user interactions. You might be interested in finding out which topics a user has posted in, has watched, or bookmarked. However, complications often arise, especially when trying to exclude ignored topics from your results.

In this guide, we’ll take a deep dive into how to efficiently extract topic_id from various tables and the common pitfalls you may encounter along the way.

Understanding the Problem

Consider the following tables within your MySQL database:

posts: Contains the records of user posts.

topics: Holds the details about each topic.

topics_watch: Tracks topics watched by users.

bookmarks: Lists topics bookmarked by users.

topics_ignore: Manages topics that users have chosen to ignore.

You’re tasked with generating a list of topic_ids grouped by user engagement while ensuring that the ignored topics are excluded from your results.

The Initial Query

You may find yourself crafting a SQL query that resembles the one below:

[[See Video to Reveal this Text or Code Snippet]]

While your query might seem successful, it doesn’t yield the expected results regarding the topics_watch and bookmarks tables.

The Solution

Removing Aggregation Functions

The main issue arises from using the GROUP BY clause without any aggregation function. In SQL, particularly with MySQL version 5.7 and later, this can lead to unpredictable results or errors. Instead, you should focus on retrieving distinct results without grouping.

Here’s an improved version of your query:

[[See Video to Reveal this Text or Code Snippet]]

Merging Results with UNION

If you want to consolidate results from the various tables— posts, watched topics, and bookmarks— you can make use of the UNION operator to merge multiple SELECT queries into a single result set. Here’s how you can do this:

[[See Video to Reveal this Text or Code Snippet]]

This approach effectively merges the topic_ids from each relevant table while maintaining the chronological order you desire.

Conclusion

Navigating SQL queries across multiple tables can be complex, but understanding the structure of your database and using the appropriate SQL techniques, such as avoiding unnecessary aggregation and leveraging UNION, can simplify the process significantly.

Now you can confidently retrieve the topic_ids that reflect a user’s engagement—without the interference of ignored topics.

By following these guidelines, you can enhance the accuracy of your queries and improve the performance of your database operations.