Oracle Interview Questions - Part 1

What are the Back ground processes in Oracle and what are they?
This is one of the most frequently asked question. There are basically 9 Processes but in a general system we need to mention the first five background processes. They do the house keeping activities for the Oracle and are common in any system.
The various background processes in oracle are
a) Data Base Writer (DBWR): Data Base Writer Writes Modified blocks from Database buffer cache to Data Files. This is required since the data is not written whenever a transaction is committed.
b)LogWriter (LGWR) : LogWriter writes the redo log entries to disk. Redo Log data is generated in redo log buffer of SGA. As transaction commits and log buffer fills, LGWR writes log entries into a online redo log file.
c) System Monitor(SMON) : The System Monitor performs instance recovery at instance startup. This is useful for recovery from system failure
d)Process Monitor(PMON) : The Process Monitor performs process recovery when user Process fails. Pmon Clears and Frees resources that process was using.
e) CheckPoint(CKPT) : At Specified times, all modified database buffers in SGA are written to data files by DBWR at Checkpoints and Updating all data files and control files of database to indicate the
most recent checkpoint
f)Archieves(ARCH) : The Archiver copies online redo log files to archival storal when they are busy.
g) Recoveror(RECO) : The Recoveror is used to resolve the distributed transaction in network
h) Dispatcher (Dnnn) : The Dispatcher is useful in Multi Threaded Architecture
i) Lckn : We can have upto 10 lock processes for inter instance locking in parallel sql.

 

How many types of Sql Statements are there in Oracle?
There are basically 6 types of sql statements. They are
a) Data Definition Language (DDL): The DDL statements define and maintain objects and drop objects.
b) Data Manipulation Language (DML): The DML statements manipulate database data.
c) Transaction Control Statements : Manage change by DML
d) Session Control : Used to control the properties of current session enabling and disabling roles and changing .e.g : Alter Statements, Set Role
e) System Control Statements : Change Properties of Oracle Instance .e.g: Alter System
f) Embedded Sql : Incorporate DDL, DML and T.C.S in Programming Language. e.g: Using the Sql Statements in languages such as 'C', Open, Fetch, execute and close

 

What is a Transaction in Oracle?
A transaction is a Logical unit of work that compromises one or more SQL Statements executed by a single User. According to ANSI, a transaction begins with first executable statement and ends when it is explicitly committed or rolled back.

 

List some Key Words Used in Oracle.
The Key words that are used in Oracle are
a) Committing: A transaction is said to be committed when the transaction makes permanent changes resulting from the SQL statements.
b) Rollback A transaction that retracts any of the changes resulting from SQL statements in Transaction.
c) SavePoint : For long transactions that contain many SQL statements, intermediate markers or savepoints are declared. Savepoints can be used to divide a transaction into smaller points.
d) Rolling Forward : Process of applying redo log during recovery is called rolling forward.
e) Cursor : A cursor is a handle ( name or a pointer) for the memory associated with a specific statement. A cursor is basically an area allocated by Oracle for executing the Sql Statement. Oracle uses an implicit cursor statement for Single row query and Uses Explicit cursor for a multi row query.
f) System Global Area (SGA): The SGA is a shared memory region allocated by the Oracle that contains Data and control information for one Oracle Instance. It consists of Database Buffer Cache and Redo log Buffer.
g) Program Global Area (PGA): The PGA is a memory buffer that contains data and control information for server process.
g) Database Buffer Cache: Database Buffer of SGA stores the most recently used blocks of database data. The set of database buffers in an instance is called Database Buffer Cache.
h) Redo log Buffer: Redo log Buffer of SGA stores all the redo log entries.
i) Redo Log Files: Redo log files are set of files that protect altered database data in memory that has not been written to Data Files. They are basically used for backup when a database crashes.
j) Process : A Process is a 'thread of control' or mechanism in Operating System that executes series of steps.

 

What are Procedure, functions and Packages?
Procedures and functions consist of set of PL/SQL statements that are grouped together as a unit to solve a specific problem or perform set of related tasks.
Procedures do not Return values while Functions return one One Value
Packages: Packages Provide a method of encapsulating and storing related procedures, functions, variables and other Package Content

 

What are Database Triggers and Stored Procedures?
Database Triggers : Database Triggers are Procedures that are automatically executed as a result of insert in, update to, or delete from table.
Database triggers have the values old and new to denote the old value in the table before it is deleted and the new indicated the new value that will be used. DT are useful for implementing complex business rules which cannot be enforced using the integrity rules. We can have the trigger as Before trigger or After Trigger and at Statement or Row level.
e.g: operations insert, update ,delete 3
before ,after 3*2 A total of 6 combinations
At statement level (once for the trigger) or row level( for every execution ) 6 * 2 A total of 12.
Thus a total of 12 combinations are there and the restriction of usage of 12 triggers has been lifted from Oracle 7.3 Onwards.
Stored Procedures : Stored Procedures are Procedures that are stored in Compiled form in the database. The advantage of using the stored procedures is that many users can use the same procedure in compiled and ready to use format.

 

How many Integrity Rules are there and what are they?
There are Three Integrity Rules. They are as follows :
a) Entity Integrity Rule : The Entity Integrity Rule enforces that the Primary key cannot be Null
b) Foreign Key Integrity Rule : The FKIR denotes that the relationship between the foreign key and the primary key has to be enforced. When there is data in Child Tables the Master tables cannot be deleted.
c) Business Integrity Rules : The Third Integrity rule is about the complex business processes which cannot be implemented by the above 2 rules.

 

What are the Various Master and Detail Relation ships?
The various Master and Detail Relationship are
a) NonIsolated : The Master cannot be deleted when a child is existing
b) Isolated : The Master can be deleted when the child is existing
c) Cascading : The child gets deleted when the Master is deleted.

 

What are the Various Block Coordination Properties?
The various Block Coordination Properties are
a) Immediate
Default Setting. The Detail records are shown when the Master Record are shown.
b) Deferred with Auto Query
Oracle Forms defer fetching the detail records until the operator navigates to the detail block.
c) Deferred with No Auto Query
The operator must navigate to the detail block and explicitly execute a query

 

What are the Different Optimization Techniques?
The Various Optimization techniques are
a) Execute Plan : we can see the plan of the query and change it accordingly based on the indexes
b) Optimizer_hint :
set_item_property('DeptBlock',OPTIMIZER_HINT,'FIRST_ROWS');
Select /*+ First_Rows */ Deptno,Dname,Loc,Rowid from dept
where (Deptno > 25)
c) Optimize_Sql :
By setting the Optimize_Sql = No, Oracle Forms assigns a single cursor for all SQL statements. This slow downs the processing because for evertime the SQL must be parsed whenever they are executed.
f45run module = my_firstform userid = scott/tiger optimize_sql = No
d) Optimize_Tp :
By setting the Optimize_Tp= No, Oracle Forms assigns separate cursor only for each query SELECT statement. All other SQL statements reuse the cursor.
f45run module = my_firstform userid = scott/tiger optimize_Tp = No

 

How do u implement the If statement in the Select Statement?
We can implement the if statement in the select statement by using the Decode statement.
e.g select DECODE (EMP_CAT,'1','First','2','Second'Null);
Here the Null is the else statement where null is done .

 

How many types of Exceptions are there?
There are 2 types of exceptions. They are
a) System Exceptions
e.g. When no_data_found, When too_many_rows
b) User Defined Exceptions
e.g. My_exception exception
When My_exception then

 

What are the inline and the precompiler directives?
The inline and precompiler directives detect the values directly

 

How do you use the same lov for 2 columns?
We can use the same lov for 2 columns by passing the return values in global values and using the global values in the code

 

How many minimum groups are required for a matrix report?
The minimum number of groups in matrix report are 4

 

What is the difference between static and dynamic lov?
The static lov contains the predetermined values while the dynamic lov contains values that come at run time

 

What are snap shots and views?
Snapshots are mirror or replicas of tables. Views are built using the columns from one or more tables. The Single Table View can be updated but the view with multi table cannot be updated

 

What are the OOPS concepts in Oracle?
Oracle does implement the OOPS concepts. The best example is the Property Classes. We can categorize the properties by setting the visual attributes and then attach the property classes for the
objects. OOPS supports the concepts of objects and classes and we can consider the property classes as classes and the items as objects

 

Privileges and Grants
Privileges are the right to execute a particular type of SQL statements.
e.g : Right to Connect, Right to create, Right to resource
Grants are given to the objects so that the object might be accessed accordingly. The grant has to be
given by the owner of the object.

 

What is the difference between candidate key, unique key and primary key?
Candidate keys are the columns in the table that could be the primary keys and the primary key
is the key that has been selected to identify the rows. Unique key is also useful for identifying the distinct rows in the table.

 

What is concurrency?
Concurrency is allowing simultaneous access of same data by different users. Locks useful for accessing the database are
a) Exclusive
The exclusive lock is useful for locking the row when an insert, update or delete is being done. This lock should not be applied when we do only select from the row.
b) Share lock
We can do the table as Share_Lock as many share_locks can be put on the same resource.

 

Table Space, Data Files, Parameter File, Control Files
Table Space : The table space is useful for storing the data in the database. When a database is created two table spaces are created.
a) System Table space : This data file stores all the tables related to the system and dba tables
b) User Table space : This data file stores all the user related tables
We should have separate table spaces for storing the tables and indexes so that the access is fast.
Data Files : Every Oracle Data Base has one or more physical data files. They store the data for the database. Every datafile is associated with only one database. Once the Data file is created the size cannot change. To increase the size of the database to store more data we have to add data file.
Parameter Files : Parameter file is needed to start an instance. A parameter file contains the list of instance configuration parameters e.g.:
db_block_buffers = 500
db_name = ORA7
db_domain = u.s.acme lang
Control Files : Control files record the physical structure of the data files and redo log files
They contain the Db name, name and location of dbs, data files ,redo log files and time stamp.

 

What is Row Chaining?
The data of a row in a table may not be able to fit the same data block. Data for row is stored in a chain of data blocks .

 

What are the Pct Free and Pct Used?
Pct Free is used to denote the percentage of the free space that is to be left when creating a table. Similarly Pct Used is used to denote the percentage of the used space that is to be used when creating a table
eg.: Pctfree 20, Pctused 40

 

Physical Storage of the Data
The finest level of granularity of the data base are the data blocks.
Data Block : One Data Block correspond to specific number of physical database space
Extent : Extent is the number of specific number of contiguous data blocks.
Segments : Set of Extents allocated for Extents. There are three types of Segments
a) Data Segment : Non Clustered Table has data segment data of every table is stored in
cluster data segment
b) Index Segment : Each Index has index segment that stores data
c) Roll Back Segment : Temporarily store 'undo' information


Comments (4)

Said this on 6-14-2009 At 12:23 pm
Very Useful . Thanks a Lot .
Said this on 2-21-2010 At 10:59 am

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Said this on 10-20-2010 At 09:33 am

how to select employee details from a table which has the salary greater than sum of salary of 2 departments in that table?

Said this on 9-14-2011 At 04:53 pm

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