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44. How do you evaluate performance ?
Using SQL TRACE. It is an utility that can monitor and report on database performance
when one or more queries are run against the database.
It is used to gather statistics when running the query (i.e) reports on CPU time spent on the
query, the total no. of rows processed and statistics related to parsing and cache performance.
53. What is a DEAD LOCK ?
A DEAD lock is a rare situation in which two or more user processes of a database cannot
complete their tansactions.This occurs because each process is holding a resource that the other
process requires (such as a row in a table) in order to complete.
63. What does `GROUP BY' statement do ?
GROUP BY statement causes a SELECT statement to produce ONE SUMMARY ROW
for all selected rows that have identical values in one or more specified column or expressions.
Each expe\ressionin the SELECT clause must be one of the following :
1] A CONSANT
2] A Function without parameters
3] A GROUP function like SUM , AVG.
4] Matched IDENTICALLY to a expression in the `GROUP BY' clause.
75. Difference between SQL and PL/SQL ?
SQL is the ANSI industry standard language, used to manipulate information in a
relational database.
PL/SQL is the procedural language extension to Oracle's SQL language.
SQL
PL/SQL
1. It is flexible, Powerful and easy to learn.
2. It is a non-procedural language. It
a] Processes set of records rather than
just one at a time and
b] Provides automatic navigation to the
data.
3. It provides commands for avariety of
tasks including :
a] Querying data
b] Creating,Updating and Replacing
objects and Inserting, Updating
and Deleting rows.
4] All RDBMS supports SQL
Thus one can transfer the skills gained
with SQL from one
RDBMS to another.
Programs written in SQL are portable,
they can often be moved from one database
to another with little modification.
1. PL/SQL block can contain any no. of
SQL statements combined with the
following
:
a] Flow of control statements such as
IF..THEN, ELSE, EXIT and GOTO.
b] Repetition statements such as FOR ..
LOOP and WHILE .. LOOP.
c] Assignment statements such as X := Y +
Z
2. PL/SQL allows you to logically group a
set of statements and send them to the
RDBMS as a single block.
3. Procedural capabilities.
4. Improved performance.
5. Enhanced productivity
6. Portability
7. Integration with the RDBMS.
77. What is the purpose of OUTER JOIN ?
An OUTER JOIN returns all the rows returned by simple join as well as those rows from
one table that do not match any row from the other table. The symbol (+) represents the outer join.
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