How Can I Allow NULL Values in a Foreign Key Column for MySQL Image Metadata?

Allowing NULL Foreign Keys for MySQL Image Metadata

Your proposed database schema for an image website faces a challenge when representing possible admin flags associated with images. In particular, you seek a way to allow for NULL values in the tblImageFlags.resolutionTypeID column, which is foreign-keyed to another table.

The Issue:

tblImageFlags.resolutionTypeID establishes a foreign key relationship with the luResolutionTypes table on the resolutionTypeID column. However, upon initially issuing a flag, it may not have a logical resolution type and thus requires a NULL value. Yet, the NOT NULL attribute on the column prevents this.

Solution:

To allow for NULL values in the foreign key column, you should remove the NOT NULL attribute from tblImageFlags.resolutionTypeID:

<code class="sql">ALTER TABLE tblImageFlags
MODIFY COLUMN resolutionTypeID INT UNSIGNED NULL;</code>

With this modification, you can now assign NULL to resolutionTypeID when a flag is first issued, while still maintaining the foreign key relationship to the luResolutionTypes table when a resolution is determined.

Bonus Points Clarification:

In the context of databases, the proper plural form of "index" is "indexes," not "indices." According to Bryan A. Garner's "Modern American Usage," "indexes" is the preferred plural for ordinary purposes, while "indices" may be permissible in technical contexts or as "indicators."

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