Скачать или смотреть Solving the MySQL Subquery Returns More Than One Row Problem in a Single Query

Discover how to simplify complex MySQL queries using aggregation for better performance and readability.
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This video is based on the question https://stackoverflow.com/q/74873334/ asked by the user 'Eduardo Luís' ( https://stackoverflow.com/u/12586112/ ) and on the answer https://stackoverflow.com/a/74873460/ provided by the user 'Tim Biegeleisen' ( https://stackoverflow.com/u/1863229/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the MySQL Challenge: Subqueries Returning Multiple Rows

As a database administrator or developer, you may often encounter situations where you need to extract information from your database with MySQL queries. A common challenge arises when you try to pull multiple results from subqueries. You might find yourself receiving an error like "Subquery returns more than one row," which can be frustrating. In this post, we will explore a specific case and demonstrate how to efficiently combine two queries into one through the use of aggregation.

The Scenario

Imagine you have a table named numeracao that records various metrics with a timestamp in the DATA column. Your goal is to retrieve two sets of data from this table:

The count of entries grouped by month.

The sum of a column named revisoes, also grouped by month.

You might start with two separate queries that yield the required information independently, but when you attempt to execute them together, you run into issues. Let's take a closer look at your original queries:

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

The reason these queries cannot be executed together is that subqueries are returning multiple rows, which is not acceptable in this context. So how can you modify this to get the results you want in one shot?

The Solution: A Single Aggregation Query

To resolve this issue, the best approach is to consolidate your two subqueries into one unified query by leveraging aggregate functions. Here’s how you can do this:

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

Breakdown of the Solution

SELECT Statement: You're selecting three pieces of information:

MONTH(DATA) AS month: This extracts the month from the DATA field.

COUNT(id) AS cnt: This counts the number of entries (or IDs) for each month.

SUM(revisoes) AS sum: This computes the total of the revisoes values for each month.

FROM Clause: This specifies the numeracao table from which the data is retrieved.

WHERE Clause: This filters the records to include only those with dates between January 1, 2022, and December 31, 2022.

GROUP BY Clause: This groups the results by month so that counts and sums apply to each individual month rather than the entire dataset.

Conclusion

By using this single aggregation query, you neatly avoid the complications that arise with subqueries returning multiple rows. Now, with a singular, powerful statement, you can efficiently gather the necessary data from your numeracao table, improving both performance and readability.

If you're dealing with a similar situation in MySQL, remember this approach for a streamlined solution!