You can't reuse the member-list of an SQL ENUM column in other tables ^100%

ENUM columns and data reuse: Understanding the limitations

As database administrators, we often find ourselves dealing with various types of data in our databases. Among these, one type that deserves special attention is ENUMs (short for "enumerated"). ENUM columns are used to restrict a cell in a table to only include one of a pre-defined set of values. They're particularly useful when working with attributes that can have a limited number of possible values.

However, there's a common misconception about ENUM columns and data reuse that we need to debunk: you can't reuse the member-list of an SQL ENUM column in other tables.

Why ENUMs are useful

Before diving into why you can't reuse ENUM values across different tables, let's quickly explore their benefits. Here are some scenarios where ENUMs shine:

• They enforce data integrity by restricting input to only valid values.
• They reduce storage requirements by not storing redundant information (e.g., multiple records with the same "status" value).
• They simplify queries and indexes since the database can directly compare these fixed, pre-defined sets of values.

The problem: Reusing ENUM values

So, why can't you reuse an ENUM's member-list in another table? It all comes down to how SQL handles these types internally. When you create an ENUM column, it gets stored as a small integer (usually 1-255) instead of the actual string value. This means that each unique ENUM value is essentially replaced by its corresponding numerical index.

Here are some key implications:

• If two tables use the same ENUM definition with the same values, they won't be identical from the database's perspective.
• Attempting to copy or import an ENUM value directly from one table into another will result in a data type mismatch because the original value isn't preserved as a string.

Consequences of misunderstanding ENUMs

Now that we've explored why you can't reuse ENUM values across tables, let's discuss the potential consequences of not understanding this limitation:

• Data loss during import/export operations
• Inconsistent query results due to different internal representations of ENUM values
• Inefficient use of storage or indexing strategies, leading to performance issues

Best practices for working with ENUMs

To avoid these pitfalls, follow these guidelines when working with ENUMs:

• Designate a central location (e.g., schema table) for all ENUM definitions.
• Use foreign keys to reference and enforce consistency across related tables.
• When importing or exporting data, ensure that ENUM values are properly mapped to their corresponding string representations.

Conclusion

ENUM columns offer significant benefits when used correctly. However, they also come with limitations that can catch even experienced developers off guard. By understanding the reasons behind these restrictions and adopting best practices for working with ENUMs, you'll be better equipped to design robust databases and avoid common pitfalls associated with data reuse. Remember: it's not just about following rules; it's about building reliable systems that withstand the test of time.