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What is aggregate function? Briefly describe it?
Aggregate functions are functions that take a collection of values as input and return a one value. SQL offers 5 built-in aggregate functions:
a) Average: avg
b) Minimum:min
c) Maximum: max
d) Total:sum
e) Count:count
Average: avg
Select avg (balance)
From account
Where branch-name='perryridge'
Count :count
Select branch-name,count(distinct customer-name)
From depositior,account
Where depositor.account-number=account.account-number
groupby branch-name
E-R Model This data model is based on real world that consists of basic objects named entities and of relationship among these objects. Entities are illustrated in a database
What is a recovery scheme? An integral part of a database system is a recovery scheme that can restore the database to the reliable state that existed before the failure.
Define Deadlock recovery measures To recover from deadlock, the cycle in the wait-for-graph should be broken. The common technique of doing this is to rollbac
What are the ways in which the variable-length records arise in database systems? Storage of many record types in a file. Record types that permit variable lengths for one o
What is the difference between WHERE and HAVING clauses?. Predicates in the WHERE clause applies to every tuple while the predicate in the HAVING clause applies to groups.
What is conceptual schema? The schemas at the view level are known as subschema that describe dissimilar views of the database.
What is Identity? Identity (or AutoNumber) is a column that automatically produces numeric values. A start and increment value can be set, but most DBA leave these at 1. A GUID
Objectives differentiate DDBMS and conventional DBMS; Describe Network topology for DDBMS; Describe the concepts of horizontal and vertical fragmentation; Descr
Candidate Key: In a relation R, a candidate key for R is a subset of the set of attributes of R, which have the differnt two properties: (1) Uniqueness: N
what is natural join?
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