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    <p>No, it's not OK. Circular references between tables are messy. See this (decade old) article: <a href="http://www.sqlmag.com/article/quering/sql-by-design-the-circular-reference" rel="noreferrer">SQL By Design: The Circular Reference</a></p> <p>Some DBMS can handle these, and with special care, but MySQL will have issues. </p> <hr> <p>The first choice is, as your design, to make one of the two FKs nullable. This allows you to solve the chicken-and-egg problem (which table should I first Insert into?).</p> <p>There is a problem though with your code. It will allow a product to have a default picture where that picture will be referencing another product!</p> <p>To disallow such an error, your FK constraint should be:</p> <pre><code>CONSTRAINT FK_products_1 FOREIGN KEY (id, default_picture_id) REFERENCES products_pictures (product_id, id) ON DELETE RESTRICT --- the SET NULL options would ON UPDATE RESTRICT --- lead to other issues </code></pre> <p>This will require a <code>UNIQUE</code> constraint/index in table <code>products_pictures</code> on <code>(product_id, id)</code> for the above FK to be defined and work properly.</p> <hr> <p>Another approach is to remove the <code>Default_Picture_ID</code> column form the <code>product</code> table and add an <code>IsDefault BIT</code> column in the <code>picture</code> table. The problem with this solution is how to allow only one picture per product to have that bit on and all others to have it off. In SQL-Server (and I think in Postgres) this can be done with a partial index:</p> <pre><code>CREATE UNIQUE INDEX is_DefaultPicture ON products_pictures (Product_ID) WHERE IsDefault = 1 ; </code></pre> <p>But MySQL has no such feature.</p> <hr> <p>A third approach, which allows you to even have both FK columns defined as <code>NOT NULL</code> is to use deferrable constraints. This works in PostgreSQL and I think in Oracle. Check this question and the answer by @Erwin: <a href="https://stackoverflow.com/questions/8394177/complex-foreign-key-constraint-in-sqlalchemy/8395021#8395021">Complex foreign key constraint in SQLAlchemy</a> (the <strong>All key columns NOT NULL</strong> Part).</p> <p>Constraints in MySQL cannot be deferrable.</p> <hr> <p>A fourth approach (which I find cleanest) is to remove the <code>Default_Picture_ID</code> column and add another table. No circular path in the FK constraints and all FK columns will be <code>NOT NULL</code> with this solution:</p> <pre><code>product_default_picture ---------------------- product_id NOT NULL default_picture_id NOT NULL PRIMARY KEY (product_id) FOREIGN KEY (product_id, default_picture_id) REFERENCES products_pictures (product_id, id) </code></pre> <p>This will also require a <code>UNIQUE</code> constraint/index in table <code>products_pictures</code> on <code>(product_id, id)</code> as in solution 1.</p> <hr> <p>To summarize, with MySQL you have two options: </p> <ul> <li><p>option 1 (a nullable FK column) with the correction above to enforce integrity correctly</p></li> <li><p>option 4 (no nullable FK columns)</p></li> </ul>
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