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Raju SudheerRaju Sudheer 

1.why we cant create many to many relationship in lookup

viruSviruS

Use lookup to just link two standard or custom objects.
Use master-detail when you want you master record to control your detail object. For e.g. if quotes is a detail under opportunity, then if you delete an opportunity, the related quotes also would be deleted.
Use master-detail even when you want to create many-many relationships. Here you would be creating a junction object, which would have two master-detail relationships.
anand k 11anand k 11
Hi Raju,

We can create Many to Many relationship with two mandatory lookup relationships.

In Master detail realtionship parent must always exist on child object

In Lookup realtionship parent does not need to exist or you can change the object.
 

In master detail you can then user rollup summaries on master object.

if you like my post then please mark as a solution  

Thanks
Anand