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Discover how to effectively retrieve the latest version of records in MySQL using the `MAX` function for versioned data.
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This video is based on the question https://stackoverflow.com/q/63021089/ asked by the user 'hotmeatballsoup' ( https://stackoverflow.com/u/5235665/ ) and on the answer https://stackoverflow.com/a/63021151/ 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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How to Use the MAX Function in MySQL for Versioned Data Querying

If you've ever worked with versioned data in MySQL, you know how challenging it can be to retrieve the most recent version of a record. In this post, we will address a common problem: getting all fields from a versioned table based on a unique identifier and the highest version number.

Understanding the Problem

Imagine a table that tracks orders, where each order can have multiple versions distinguished by their order_version field. For example, one order reference (order_ref_id) can have different versions (0, 1, 2, etc.). You may want to retrieve the entire record for the latest version associated with a specific order_ref_id.

Here is what our orders table might look like:

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

In this case, we want to obtain the record with the highest order_version for the order_ref_id '12345', specifically returning:

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

The Challenge with MAX()

You may have tried a query like this:

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

However, this approach leads to an error message stating:
ERROR 1111 (HY000): Invalid use of group function. The reason is that the MAX() function cannot be used directly in the WHERE clause like this.

The Solution: Using a Subquery

To solve this problem, we can utilize a subquery that groups the records by order_ref_id while obtaining the maximum version number. Here's how to formulate the query correctly:

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

Breaking Down the Solution

Subquery:

We first write a subquery that retrieves the order_ref_id and the MAX(order_version) grouped by order_ref_id. This will identify the highest version for each set of order references.

JOIN Operation:

Then, we join this result to the orders table, matching the order_ref_id and the corresponding maximum version. This JOIN operation allows us to retrieve all fields for the record with the latest version.

Result:

By running this entire query, you will receive a complete set of fields (including order_id, order_name, etc.) for the latest version of each order based on the specified order_ref_id.

Conclusion

With this approach, you can effortlessly fetch complete records for versioned data in MySQL. Remember that by utilizing subqueries and joins effectively, you can overcome limitations posed by the MAX() function's restrictions in the WHERE clause.

Happy querying! If you have further questions or need clarification on any of the steps, feel free to ask!