Re: Foreign key problem

From: Tony Andrews <>
Date: 13 Jun 2006 02:08:52 -0700
Message-ID: <> wrote:
> I am working on an application which need a data model that reflects
> the schema structure. My tables are ("*" indicates primary key column):
> tableinfo(*table_name)
> columninfo(*table_name, *col_name)
> fk(*fk_name, pk_table, fk_table)
> fk_column(*fk_name, *fk_column, pk_column)
> My problem is the last table, fk_column. How can I define a foreign key
> for fk_column.fk_column (and pk_column) referencing the table
> columninfo? The column table_name is not an member of the table
> fk_column, but it could easily be joined from the fk table, but I don't
> think that helps me with defining the foreign key constraint?
> The corresponding information_schema tables in MySQL looks like this
> (simplified):
> table_constraints(*constraint_name, table_name)
> key_column_usage(*contraint_name, *table_name, *column_name,
> referenced_table_name, referenced_column_name)
> In this case it is possible to define foreign key constraints, but I
> don't consider these tables normalized. The value of table_name is
> given by the constraint_name and should not be repeated in the second
> table.
> Somebody that have a pice of advice?
> Fredrik Bertilsson

Your fk_column table *needs* table_name because it is part of the primary key of the parent table columninfo. This is not denormalisation, any more than having column_name in 2 tables is denormalisation, because the table_name is not redundant: it cannot be derived from the column_name alone. Add it with impunity! Received on Tue Jun 13 2006 - 11:08:52 CEST

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