Question

Today, I’ve had some debate with my colleague about choosing data types in our projects.

We’re web developers and we code back-end in PHP, and for database we use mySQL.
So, I went on internet a bit, and they don’t recommend ENUM data type for various reasons (also I’ve read here on SO that this is not recommended) – For ENUM(‘yes’,’no’) for example you should use tinyint(1) .
If ENUMs are bad and should be avoided, why does vBulletin for example, uses them?
Why use them at all when you can use VARCHAR, TEXT and so on and enforce use of 1 of 2 possible values in PHP.
Thank you for your answers.

Answer 1

Enums aren’t ideal, but they are waaaay better than your alternative suggestion of using a VARCHAR and enforcing one of a few possible values!

Enums store their data as a numeric value. This is ideal for storing a field with a limited set of possible values such as ‘yes’ or ‘no’, because it uses the minimum amount of space, and gives the quickest possible access, especially for searches.

Where enums fall over is if you later need to add additional values to the list. Let’s say you need to have ‘maybe’ as well as ‘yes’ or ‘no’. Because it’s stored in an enum, this change requires a database change. This is a bad thing for several reasons – for example, if you have a large data set, it can take a significant amount of time to rebuild the table.

The solution to this is to use a related table which stores a list of possible values, and your original field would now simply contain an ID reference to your new table, and queries would make a join to the lookup table to get the string value. This is called “normalisation” and is considered good database practice. It’s a classic relational database scenario to have a large number of these lookup tables.

Obviously, if you care fairly sure that the field will never store anything other than ‘yes’ or ‘no’, then it can be overkill to have a whole extra table for it and an enum may be appropriate.

Some database products do not even provide an enum data type, so if you’re using these DBs, you are forced to use the lookup table solution (or just a simple numeric field, and map the values in your application).

What is never appropriate in this situation is to use an actual string value in the table. This is considered extremely poor practice.

VARCHARS take up much more disk space than the numeric values used by an enum. They are also slower to read, and slower to look up in a query. In addition, they remove the enforcement of fixed values provided by enum. This means that a bug in your program could result in invalid values going into the data, as could an inadvertant update using PHPMyAdmin or a similar tool.

I hope that helps.