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Explain mechanism of nested queries?
SQL gives a mechanism for nesting subqueries. A subquery is a select from where expression that is nested within one more query. A common use of sub queries is to perform tests for set membership, make set comparisons, and verify set cardinality.
a) Set membership
(select customer-name
from depositor)
b) Set comparison
select distinct T.branch-name
from branch as T,branch as S
where T.assets > S.assets and S.branch-city='Brooklyn'
a) Test for Empty Relations
Select customer-name
from borrower
where exits (select *
from depositor
where depositor.customername=borrower.customer-name)
a) Test for the Absence of Duplicate Tuples
Dirty Reads: T10 reads a value which is updated by T9. This update has not been committed and T9 aborts. T9 T10 Value of x old value = 200
Lost Updates : Suppose the two transactions T3 and T4 run simultaneously and they happen to be interleaved in the following way (suppose the initial value of X as 10000):
List the types of authorization. A) Read authorization B) Write authorization C) Update authorization D) Drop authorization
Describe log based recovery The most usably structure for recording data base modification is the LOG the log is a sequence of log records recording all the update activities i
Representing complex relationship among data A database may contain varieties of data interrelated to each other in numerous ways. A DBMS must have the capability to represent
what is binary relationship
In which relational modes, cardinality is termed? Answer : Cardinality is termed in a number of tuples relational modes.
What is Index? An index is a physical structure having pointers to the data. Indices are created in an existing table to locate rows more quickly and efficiently. It is possibl
When is UPDATE_STATISTICS command used? - When processing of large data is done, this command is used. - Whenever large number of modification, deletions or copy takes place
Define Database layer? The database layer defines the data formats used in the database.
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