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ER Notation

Default Notation

Our default simplified Entity-Relationship notation tries to reduce clutter and stick to the essential in the visual representation. Query diagrams can show only this special notation. Model diagrams may also show other notations. To change default notation, switch the choice of Model Notation option.

Optional relationships appear with a dash line. This means the foreign key(s), usually on the to-many side, could be NULL. All custom relationships are also optional.

We show only the to-many cardinality, as a crow foot, at one end or both ends of the connector.

notation-model-xtractor

Query diagrams may represent inclusive and exclusive outer joins with one small black or empty circle at one or both ends.

Crow’s Foot Notation

Crow’s Foot or IE (Information Engineering) notation could be the most popular standard ER notation for relational data modeling.

Our Crow’s Foot notation properly shows all reverse-engineered to-many relationships as to-zero-or-many, never as to-one-or-many. This is because no RDBMS can enforce through its metadata alone a parent-child relationship to be mandatory.

At the other end, minimum cardinality could be zero-or-one, or one. We do not show one-and-only-one, with two parallel segment, to eliminate clutter.

notation-crows-foot

Identifying relationships appear with a plain line, non-identifying are dashed. Identifying relationships are usually from an independent table (with round corners) to a dependent table (with square corners), which has the propagated foreign key part of its primary key. All child table rows have a related parent row in the independent table.

IDEF1X Notation

Integration DEFinition for Information Modeling is the default notation in data modeling tools such as ERwin.

To-many appears as a black circle, from zero-or-one as a small empty diamond. All other clutter is excluded.

notation-idef1x

Same previous rules for identifying and non-identifying relationships apply. Dash lines show non-identifying relationships. Remark all intersection tables, whose primary keys are the combination of propagated foreign keys, show always a plain line. Reflexive relationships are always optional (at least one entry’s foreign key is NULL) and from-zero-or-one. One-to-zero-or-one are always identifying, the optional cardinality at one end having the primary key as propagated foreign key.

Simplified Notation

With diagram’s option Show Simplified on, all clutter is removed. What matters and what we show, for any expanded relationship, is only a plain line between every two shapes. No dash line and no specific connector headers:

notation-simplified

This mode may be useful in topological and graph models, when you want to surprise the overall connectivity of the model.

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