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Let´s say I have 3 entities. Can I have them all linked to each other "forming a circle" where each one is linked to the other trough a many to many relationship.

Cheers

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You need to provide more information. Of course you can have them all 'linked.' The real question is, should you? And there's no way to say that unless you give more information about the logical design of your database. What data is actually being modeled in this case? –  Jon Boulineau Nov 1 '13 at 12:07
    
Are you using ER modeiling to capture analysis or design? If you are analyzing the subject matter, and you discover circular relationships in the real world, by all means you should model them. If you are designing a database, then you need to consider whether all of these relationships are going to be reflected in the design. –  Walter Mitty Nov 4 '13 at 13:16

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Sometimes this is called "a cycle" in database design, and while it is not forbidden (because as the commenters point out, you must take into account your purpose and business problem), it is considered a red flag for many designers.

Here is a related Stack Overflow question that attempts to point out why this can be a red flag: http://stackoverflow.com/questions/8115748/why-should-i-avoid-loops-when-designing-relationships-for-a-database

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