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Views in relational algebra:
1. Basic expression contains either
a) A relation in the database.
b) A constant relation.
2. General expressions are madde out of smaller sub expressions using
a) σp(E1) select (p a predicate)
b) ∏s(E1) project (s a list of attributes)
c) Px(E1)rename (x a relation name)
d) E1∪E2 union
e) E1- E2 set difference
f) E1x x E2 cartesian product
Define lock? Lock is the most common used to execute the requirement is to permit a transaction to access a data item only if it is currently holding a lock on that item.
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