Oracle Interview Questions and Answers -2

Oracle Interview Questions and Answers

What is the advantage of a stored procedure over a database trigger ?
We have control over the firing of a stored procedure but we have no control over the firing of a trigger.

What is the maximum no. of statements that can be specified in a trigger statement ?
One.

Can views be specified in a trigger statement ?
No

What are the values of :new and :old in Insert/Delete/Update Triggers ?
INSERT : new = new value, old = NULL
DELETE : new = NULL, old = old value
UPDATE : new = new value, old = old value

What are cascading triggers? What is the maximum no of cascading triggers at a time?
When a statement in a trigger body causes another trigger to be fired, the triggers are said to be cascading. Max = 32.

What are mutating triggers ?
A trigger giving a SELECT on the table on which the trigger is written.

What are constraining triggers ?
A trigger giving an Insert/Update on a table having referential integrity constraint on the triggering table.

Describe Oracle database's physical and logical structure ?
Physical : Data files, Redo Log files, Control file.
Logical : Tables, Views, Tablespaces, etc.

Can you increase the size of a tablespace ? How ?
Yes, by adding datafiles to it.

What is the use of Control files ?
Contains pointers to locations of various data files, redo log files, etc.

What is the use of Data Dictionary ?
Used by Oracle to store information about various physical and logical Oracle structures e.g. Tables, Tablespaces, datafiles, etc

What are the advantages of clusters ?
Access time reduced for joins.

What are the disadvantages of clusters ?
The time for Insert increases.

Can Long/Long RAW be clustered ?
No.

Can null keys be entered in cluster index, normal index ?
Yes.

Can Check constraint be used for self referential integrity ? How ?
Yes. In the CHECK condition for a column of a table, we can reference some other column of the same table and thus enforce self referential integrity.

What are the min. extents allocated to a rollback extent ?
Two

What are the states of a rollback segment ? What is the difference between partly available and needs recovery ?
The various states of a rollback segment are :
ONLINE, OFFLINE, PARTLY AVAILABLE, NEEDS RECOVERY and INVALID.

What is the difference between unique key and primary key ?
Unique key can be null; Primary key cannot be null.

An insert statement followed by a create table statement followed by rollback ? Will the rows be inserted ?
No.

Can you define multiple savepoints ?
Yes.

What is the maximum no. of columns a table can have ?
254. (In ORACLE 8)

What is the significance of the & and && operators in PL SQL ?
The & operator means that the PL SQL block requires user input for a variable. The && operator means that the value of this variable should be the same as inputted by the user previously for this same variable. If a transaction is very large, and the rollback segment is not able to hold the rollback information, then will the transaction span across different rollback segments or will it terminate ? It will terminate (Please check ).

Can you pass a parameter to a cursor ?
Explicit cursors can take parameters, as the example below shows. A cursor parameter can appear in a query wherever a constant can appear. CURSOR c1 (median IN NUMBER) IS SELECT job, ename FROM emp WHERE sal > median;

What are the various types of RollBack Segments ?
Public Available to all instances
Private Available to specific instance

Can you use %RowCount as a parameter to a cursor ?
Yes

Is the query below allowed :
Select sal, ename Into x From emp Where ename = 'KING'
(Where x is a record of Number(4) and Char(15))
Yes

How many rows will the following SQL return :
Select * from emp Where rownum < 10;
9 rows

Definition of relational DataBase by Dr. Codd (IBM)?
A Relational Database is a database where all data visible to the user is organized strictly as tables of data values and where all database operations work on these tables.

What is Multi Threaded Server (MTA) ?
In a Single Threaded Architecture (or a dedicated server configuration) the database manager creates a separate process for each database user. But in MTA the database manager can assign multiple users (multiple user processes) to a single dispatcher (server process), a controlling process that queues request for work thus reducing the databases memory requirement and resources.

Which are initial RDBMS, Hierarchical & N/w database ?
RDBMS - R system
Hierarchical - IMS
N/W - DBTG

What is Functional Dependency
Given a relation R, attribute Y of R is functionally dependent on attribute X of R if and only if each X-value has associated with it precisely one -Y value in R

What is Auditing ?
The database has the ability to audit all actions that take place within it.
a) Login attempts, b) Object Accesss, c) Database Action Result of Greatest(1,NULL) or Least(1,NULL) NULL

When to create indexes ?
To be created when table is queried for less than 2% or 4% to 25% of the table rows.

How can you avoid indexes ?
TO make index access path unavailable - Use FULL hint to optimizer for full table scan - Use INDEX or AND-EQUAL hint to optimizer to use one index or set to indexes instead of another. - Use an expression in the Where Clause of the SQL.

What is the result of the following SQL :
Select 1 from dual
UNION
Select 'A' from dual;
Error

Can database trigger written on synonym of a table and if it can be then what would be the effect if original table is accessed.
Yes, database trigger would fire.

Can you alter synonym of view or view ?
No

Can you create index on view ?
No

What is the difference between a view and a synonym ?
Synonym is just a second name of table used for multiple link of database. View can be created with many tables, and with virtual columns and with conditions. But synonym can be on view.

What is the difference between alias and synonym ?
Alias is temporary and used with one query. Synonym is permanent and not used as alias.

What are the advantages of Oracle ?
Portability : Oracle is ported to more platforms than any of its competitors, running on more than 100 hardware platforms and 20 networking protocols.
Market Presence : Oracle is by far the largest RDBMS vendor and spends more on R & D than most of its competitors earn in total revenue. This market clout means that you are unlikely to be left in the lurch by Oracle and there are always lots of third party interfaces available.
Backup and Recovery : Oracle provides industrial strength support for on-line backup and recovery and good software fault tolerence to disk failure. You can also do point-in-time recovery.
Performance : Speed of a 'tuned' Oracle Database and application is quite good, even with large databases. Oracle can manage > 100GB databases.
Multiple database support : Oracle has a superior ability to manage multiple databases within the same transaction using a two-phase commit protocol.

What is a forward declaration ? What is its use ?
PL/SQL requires that you declare an identifier before using it. Therefore, you must declare a subprogram before calling it. This declaration at the start of a subprogram is called forward declaration. A forward declaration consists of a subprogram specification terminated by a semicolon.

What are actual and formal parameters ?
Actual Parameters : Subprograms pass information using parameters. The variables or expressions referenced in the parameter list of a subprogram call are actual parameters. For example, the following procedure call lists two actual parameters named emp_num and amount:
Eg. raise_salary(emp_num, amount);
Formal Parameters : The variables declared in a subprogram specification and referenced in the subprogram body are formal parameters. For example, the following procedure declares two formal parameters named emp_id and increase: Eg. PROCEDURE raise_salary (emp_id INTEGER, increase REAL) IS current_salary REAL;

What are the types of Notation ?
Position, Named, Mixed and Restrictions.

What all important parameters of the init.ora are supposed to be increased if you want to increase the SGA size ?
In our case, db_block_buffers was changed from 60 to 1000 (std values are 60, 550 & 3500) shared_pool_size was changed from 3.5MB to 9MB (std values are 3.5, 5 & 9MB) open_cursors was changed from 200 to 300 (std values are 200 & 300) db_block_size was changed from 2048 (2K) to 4096 (4K) {at the time of database creation}.
The initial SGA was around 4MB when the server RAM was 32MB and The new SGA was around 13MB when the server RAM was increased to 128MB.

If I have an execute privilege on a procedure in another users schema, can I execute his procedure even though I do not have privileges on the tables within the procedure ?
Yes

What are various types of joins ?
Equijoins, Non-equijoins, self join, outer join

What is a package cursor ?
A package cursor is a cursor which you declare in the package specification without an SQL statement. The SQL statement for the cursor is attached dynamically at runtime from calling procedures.

If you insert a row in a table, then create another table and then say Rollback. In this case will the row be inserted ?
Yes. Because Create table is a DDL which commits automatically as soon as it is executed. The DDL commits the transaction even if the create statement fails internally (eg table already exists error) and not syntactically.

What are the various types of queries ??
Normal Queries
Sub Queries
Co-related queries
Nested queries
Compound queries

What is a transaction ?
A transaction is a set of SQL statements between any two COMMIT and ROLLBACK statements.

What is implicit cursor and how is it used by Oracle ?
An implicit cursor is a cursor which is internally created by Oracle. It is created by Oracle for each individual SQL.

Which of the following is not a schema object : Indexes, tables, public synonyms, triggers and packages ?
Public synonyms

What is PL/SQL?
PL/SQL is Oracle's Procedural Language extension to SQL. The language includes object oriented programming techniques such as encapsulation, function overloading, information hiding (all but inheritance), and so, brings state-of-the-art programming to the Oracle database server and a variety of Oracle tools.

Can one read/write files from PL/SQL?
Included in Oracle 7.3 is a UTL_FILE package that can read and write files. The directory you intend writing to has to be in your INIT.ORA file (see UTL_FILE_DIR=... parameter). Before Oracle 7.3 the only means of writing a file was to use DBMS_OUTPUT with the SQL*Plus SPOOL command.
DECLARE
fileHandler UTL_FILE.FILE_TYPE;
BEGIN
fileHandler := UTL_FILE.FOPEN('/home/oracle/tmp', 'myoutput','W');
UTL_FILE.PUTF(fileHandler, 'Value of func1 is %sn', func1(1));
UTL_FILE.FCLOSE(fileHandler);
END;

How can I protect my PL/SQL source code?
PL/SQL V2.2, available with Oracle7.2, implements a binary wrapper for PL/SQL programs to protect the source code. This is done via a standalone utility that transforms the PL/SQL source code into portable binary object code (somewhat larger than the original). This way you can distribute software without having to worry about exposing your proprietary algorithms and methods. SQL*Plus and SQL*DBA will still understand and know how to execute such scripts. Just be careful, there is no "decode" command available.
The syntax is:
wrap iname=myscript.sql oname=xxxx.yyy

Can one use dynamic SQL within PL/SQL? OR Can you use a DDL in a procedure ? How ?
From PL/SQL V2.1 one can use the DBMS_SQL package to execute dynamic SQL statements.
Eg: CREATE OR REPLACE PROCEDURE DYNSQL
AS
cur integer;
rc integer;
BEGIN
cur := DBMS_SQL.OPEN_CURSOR;
DBMS_SQL.PARSE(cur,'CREATE TABLE X (Y DATE)', DBMS_SQL.NATIVE);
rc := DBMS_SQL.EXECUTE(cur);
DBMS_SQL.CLOSE_CURSOR(cur);
END;