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Mohammad Asim AliMohammad Asim Ali 

Validation rule at object level to check if entered text is present as another field value in another object

I believe this is related to cross-object validation. Is it possible to create a validation rule on ObjA, which will validate a value of field ObjA.FieldA whether any record exists in ObjB with the same value in ObjB.FieldB?

I can't use apex code because ObjB has more than 50 million records. Currently there is no relationship between the two objects, but I can create one based on suggestions from you experts.
Mohammad Asim AliMohammad Asim Ali
After some research I relaized that VLOOKUP is the correct way to go about it. Unfortunately in my case, ObjB is a standard field and VLOOKUP doesn't support standard objects.

For future reference the validation rule on Obj A would look something like this - 

ISNULL(VLOOKUP($ObjectType.ObjB.Fields.FieldB, $ObjectType.ObjB.Fields.Name, FieldA))

But ObjB should be a custom object.