Mastering the Aggregate Query in MySQL: Displaying the Highest Value of Non-Aggregated Columns

Learn how to aggregate queries in MySQL efficiently, displaying the highest value of non-aggregated columns using recent SQL techniques like row_number.
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Mastering Aggregate Queries in MySQL: Displaying the Highest Value of Non-Aggregated Columns

In the world of data management, particularly when using SQL databases like MySQL, we often encounter complex data retrieval tasks. One common question that arises is how to perform an aggregate query while also displaying the highest value of non-aggregated columns. In this guide, we will dive into this topic, providing context on the query structure and showcasing how to achieve the desired results efficiently.

The Problem: Aggregating Queries with Highest Values

You may find yourself in situations where you need to group data and retrieve specific details based on certain criteria. For instance, let's say we have a dataset containing information about employee salaries with effective dates. Our goal is to group these records by a salary element (denoted as salElem) while retrieving the information (like importo) associated with the most recent effective date (datainizio) for that salary element.

Example Scenario

Consider the following query that retrieves all salary elements for an employee:

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

Desired Output

The original output, however, lists all records, but we only want to see the most recent date for each salary element, like so:

CodDipsalElemdatainizioimporto912021-11-011314.19922019-11-040.00952019-11-040.00962019-11-040.00The Solution: Using Row Number in MySQL

Starting from MySQL 8 or later, you can efficiently retrieve the highest value for each group using the row_number window function. Here’s how you can structure your query:

Step-by-Step Query Construction

Common Table Expression (CTE): Create a CTE that includes a row_number for each group partitioned by salElem and ordered by datainizio in descending order.

Main Query: Select from the CTE, filtering for only those rows where rn equals 1.

Here’s the complete SQL query:

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

Explanation of Key Components:

WITH cte AS (...): This defines a temporary result set where calculations like row_number can be performed.

ROW_NUMBER() OVER (PARTITION BY ... ORDER BY ...): This function assigns a unique sequential integer to rows within the same group of salElem, sorted by datainizio descending.

Filtering with WHERE rn = 1: After assigning row numbers, we only select rows that are the first in their respective groups, which will have the latest datainizio.

Conclusion

By utilizing the row_number window function in MySQL, you can effectively tackle the challenge of aggregating queries while displaying the highest value of non-aggregated columns. This allows for cleaner, more efficient SQL queries that return precisely the information you need.

Next time you need to group data and pull the latest or highest values, remember this method—it can save you time and simplify your SQL logic!