A table can contain multiple FOREIGN KEY constraints.If a FOREIGN KEY constraint already exists, you can modify or delete it.For example, a column containing a product price should probably only accept positive values.
Thus NO ACTION is taken after updating my primary keys columns.
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The essential syntax for a foreign key constraint definition in a value is ignored if there is already an explicitly defined index on the child table that can support the foreign key.
It is possible to implement other semantics using triggers.
Additionally, My SQL requires that the referenced columns be indexed for performance reasons.