Entity Relationship Diagrams

[@decomplexify]

CORRECTIONS: Expand this comment to see corrections. CONTENTS 0:00 Introduction 0:57 Extracting information requirements 2:58 Relationships 4:07 Cardinality 6:42 Basics of Chen notation 9:50 Attributes 11:04 Weak entities 13:17 Crow's foot notation 15:10 M-M / 1-M / 1-1 relationships 18:07 From ERD to relational database 20:00 Conclusion * CORRECTIONS * In the part of the video dealing with "weak entities", at 12:06, I've underlined the Line Sequence Number attribute with a solid line. By convention, an attribute like Line Sequence Number (i.e. the attribute that, in combination with its parent's unique identifier, identifies the weak entity) should actually be underlined with a dotted line.

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[@Skyb0rg]

How would you enforce a one to many relationship in the other direction in SQL? Ex. enforce that every Address is associated with at least one Person.

[@decomplexify]

That's a little complicated. On most database platforms, you'll need to write "triggers", for example a trigger to ensure that if an attempt is made to put through a transaction that deletes the only remaining person associated with a given address, this attempt is detected and the transaction is automatically rolled back. If you are designing your database using a sophisticated data modelling tool like ERwin, the tool will be able to see (from the cardinalities you specified on the relationship in your diagram) that these triggers are needed, and will script them for you automatically.

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