Understanding MySQL Enums: Unveiling Surprising Behaviors
Published: 2023-10-27 10:20:05


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Have you ever encountered MySQL enums in your database work and been left puzzled by their behavior? In this blog, we'll delve into MySQL enums, what they are, and explore some of the intriguing and, at times, counterintuitive aspects of how they work.

### What Are MySQL Enums?
Let's start with the basics. MySQL enums are a column type that allows you to define a set list of values. From a human perspective, you can view this column as a list of strings. However, beneath the surface, the database treats them as integers, resulting in compact storage on disk. This dual nature provides a hint of data validation, as you can only choose from the predefined list of values.

### The Right Down-the-Middle Use Case
To illustrate a straightforward use case, imagine an "orders" table with a "size" column, which is defined as an enum. The allowed values for this column are: Extra Small, Small, Medium, Large, and Extra Large. These are the only options you can select, ensuring data integrity.

### Unveiling the Integer-String Duality
Now, here's where it gets interesting. MySQL enums offer both the readability of strings and the efficiency of integers. To prove this, you can perform a simple query: `SELECT size` and `SELECT size + 0`. The former will return the string label (e.g., "Small"), while the latter will coerce it into the underlying integer (e.g., 2 for "Small"). This clever design gives you the best of both worlds.

### Ordering by Enums
One intriguing aspect of enums is their behavior when it comes to sorting or ordering. When you use `ORDER BY` on an enum column, it sorts by the underlying integer values, not the string labels. This may initially seem odd, as you might expect strings to be ordered alphabetically. However, it's essential to remember that the database operates based on integers, not strings.

For instance, if you order the "size" column, you'll notice that "Extra Small" (1), "Small" (2), "Large" (4), and "Extra Large" (5) are sorted based on their integer positions. This behavior might feel counterintuitive, but it's crucial to know when working with enum columns.

### The Most Surprising Quirk: Max and Min Functions
Here comes the real surprise. While ordering by an enum column follows the integer values, the `MAX` and `MIN` functions take an unexpected approach. Instead of considering the integer positions, these functions base their calculations on the string values.

For example, if you were to run `SELECT MAX(size) FROM orders`, you might expect it to return "Extra Large" since it's in position 5. However, the result will be "Extra Small" because, alphabetically, it's the largest string label.

### Wrapping It Up
In summary, MySQL enums can be incredibly useful for specific use cases where you need to maintain a specific order of values. However, it's crucial to declare the enum values in the order you want them sorted.

The quirky behavior of enums, where ordering follows integers and max/min functions consider string values, may seem inconsistent, but it's an essential aspect to keep in mind when working with these data types. Understanding these nuances will help you make the most of MySQL enums in your database management endeavors.