Mysql - referencing one foreign key to multiple possible primary keys?

Your tables points and lines should contain a foreign key to metadata – not the other way around. Doing so will save you from defining any more complicated table setups. Using this approach, a single metadata -entry could be re-used several times for many different points or lines.

This isn't even MySQL specific but a general, normalized database structure.

Your case appears to be yet another instance of the design pattern known as "generalization specialization" or perhaps "table design for class inheritance".

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