What is meant by a safe expression in relational calculus?

1.What is meant by a safe expression in relational calculus?

2.When is a query language called relationally complete?

3.Why must the insert I. and delete D. operators of QBE appear under the relation name in a relation template, not under a column name?

4.Why must the update U. operators of QBE appear under a column name in a relation template, not under the relation name?

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