True or False :
1- If a relation has several Candidate keys, one is chosen arbitrarily to be the primary key.
2-In relational database, a special NULL value is used to represent values that are unknown or inapplicable to certain tuples.
3- Key constraints should not be violated by the Update operations (Insert, Delete, Modify).
4- the network model is based on the concept of relation.
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Answer: true,false,false,true
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