Monday, February 15, 2016

Oracle PL/SQL Interview Questions and Answers

Explain the difference between trigger and stored procedure.
Trigger in act which is performed automatically before or after a event occur
Stored procedure is a set of functionality which is executed when it is explicitly invoked.

Explain Row level and statement level trigger.
Row-level: - They get fired once for each row in a table affected by the statements.
Statement: - They get fired once for each triggering statement.

Advantage of a stored procedure over a database trigger
Firing of a stored procedure can be controlled whereas on the other hand trigger will get fired whenever any modification takes place on the table.

What are cascading triggers?
A Trigger that contains statements which cause invoking of other Triggers are known as cascading triggers. Here’s the order of execution of statements in case of cascading triggers:
·         Execute all BEFORE statement triggers that apply to the current statement.
·         Loop for each row affected statement.
·         Execute all BEFORE row triggers that apply to the current statement in the loop.
·         Lock and change row, perform integrity constraints check; release lock.
·         Execute all AFTER row triggers that apply to the current statement.
·         Execute all AFTER statement triggers that apply to the current statement.

What is a JOIN? Explain types of JOIN in oracle.
A JOIN is used to match/equate different fields from 2 or more tables using primary/foreign keys. Output is based on type of Join and what is to be queries i.e. common data between 2 tables, unique data, total data, or mutually exclusive data.
Types of JOINS:
JOIN Type
Example
Description
Simple JOIN
SELECT p.last_name,t.deptName
FROM person p, dept t
WHERE p.id = t.id;
Find name and department name of students who have been allotted a department
Inner/Equi/Natural JOIN

SELECT * from Emp INNER JOIN Dept WHERE Emp.empid=Dept.empid
Extracts data that meets the JOIN conditions only. A JOIN is by default INNER unless OUTER keyword is specified for an OUTER JOIN.
Outer Join

SELECT distinct * from Emp LEFT OUTER JOIN Dept Where Emp.empid=Dept.empid
It includes non matching rows also unlike Inner Join.
Self JOIN

SELECT a.name,b.name from emp a, emp b WHERE a.id=b.rollNumber
Joining a Table to itself.

What is object data type in oracle?
New/User defined objects can be created from any database built in types or by their combinations. It makes it easier to work with complex data like images, media (audio/video). An object types is just an abstraction of the real world entities. An object has:
·         Name
·         Attributes
·         Methods
Example:
Create type MyName as object (first varchar2(20), second varchar2(20));

Now you can use this datatype while defining a table below: 

Create table Emp (empno number(5),Name MyName);
One can access the Atributes as Emp.Name.First and Emp.Name.Second

What is composite data type?
Composite data types are also known as Collections .i.e RECORD, TABLE, NESTED TABLE, VARRAY.
Composite data types are of 2 types:
PL/SQL RECORDS
PL/SQL Collections- Table, Varray, Nested Table

Differences between CHAR and NCHAR in Oracle
NCHAR allow storing of Unicode data in the database. One can store Unicode characters regardless of the setting of the database characterset

Differences between CHAR and VARCHAR2 in Oracle
CHAR is used to store fixed length character strings where as Varchar2 can store variable length character strings. However, for performance sake Char is quit faster than Varchar2.
If we have char name[10] and store “abcde”, then 5 bytes will be filled with null values, whereas in case of varchar2 name[10] 5 bytes will be used and other 5 bytes will be freed.

Differences between DATE and TIMESTAMP in Oracle
Date is used to store date and time values including month, day, year, century, hours, minutes and seconds. It fails to provide granularity and order of execution when finding difference between 2 instances (events) having a difference of less than a second between them.
TimeStamp datatype stores everything that Date stores and additionally stores fractional seconds.
Date: 16:05:14
Timestamp: 16:05:14:000

Define CLOB and NCLOB datatypes.
CLOB: Character large object. It is 4GB in length.
NCLOB: National Character large object. It is CLOB datatype for multiple character sets , upto 4GB in length.

What is the BFILE datatypes?
It refers to an external binary file and its size is limited by the operating system.

What is Varrays?
Varrays are one-dimensional, arrays. The maximum length is defined in the declaration itself. These can be only used when you know in advance about the maximum number of items to be stored.
For example: One person can have multiple phone numbers. If we are storing this data in the tables, then we can store multiple phone numbers corresponding to single Name. If we know the maximum number of phone numbers, then we can use Varrays, else we use nested tables.

What is a cursor? What are its types?
Cursor is used to access the access the result set present in the memory. This result set contains the records returned on execution of a query.
They are of 2 types:
1.      Explicit
2.      Implicit

Explain the attributes of implicit cursor
  1. %FOUND - True, if the SQL statement has changed any rows.
  2. %NOTFOUND - True, if record was not fetched successfully.
  3. %ROWCOUNT - The number of rows affected by the SQL statement.
  4. %ISOPEN - True, if there is a SQL statement being associated to the cursor or the cursor is open.

Explain the attributes of explicit cursor.
  1. %FOUND - True, if the SQL statement has changed any rows.
  2. %NOTFOUND - True, if record was not fetched successfully.
  3. %ROWCOUNT - The number of rows affected by the SQL statement.
  4. %ISOPEN - True, if there is a SQL statement being associated to the cursor or the cursor is open.

What is the ref cursor in Oracle?
REF_CURSOR allows returning a recordset/cursor from a Stored procedure.
It is of 2 types:
Strong REF_CURSOR: Returning columns with datatype and length need to be known at compile time.
Weak REF_CURSOR: Structured does not need to be known at compile time.

Syntax till Oracle 9i
create or replace package REFCURSOR_PKG as
TYPE WEAK8i_REF_CURSOR IS REF CURSOR;
TYPE STRONG REF_CURSOR IS REF CURSOR RETURN EMP%ROWTYPE;
end REFCURSOR_PKG;
Procedure returning the REF_CURSOR:
create or replace procedure test( p_deptno IN number , p_cursor OUT
REFCURSOR_PKG.WEAK8i_REF_CURSOR)
is
begin
open p_cursor FOR
select *
from emp
where deptno = p_deptno;
end test;
 
Since Oracle 9i we can use SYS_REFCURSOR
create or replace procedure test( p_deptno IN number,p_cursor OUT SYS_REFCURSOR)
is
begin
open p_cursor FOR
select *
from emp
where deptno = p_deptno;
end test;
 
For Strong
create or replace procedure test( p_deptno IN number,p_cursor OUT REFCURSOR_PKG.STRONG
REF_CURSOR)
is
begin
open p_cursor FOR
select *
from emp
where deptno = p_deptno;
end test;

What are the drawbacks of a cursor?
Cursors allow row by row processing of recordset. For every row, a network roundtrip is made unlike in a Select query where there is just one network roundtrip. Cursors need more I/O and temp storage resources, thus it is slower.

What is a cursor variable?
In case of a cursor, Oracle opens an anonymous work area that stores processing information. This area can be accessed by cursor variable which points to this area. One must define a REF CURSOR type, and then declare cursor variables of that type to do so.
E.g.:
/* Create the cursor type. */
TYPE company_curtype IS REF CURSOR RETURN company%ROWTYPE;

/* Declare a cursor variable of that type. */
company_curvar company_curtype;
 

What is implicit cursor in Oracle?
PL/SQL creates an implicit cursor whenever an SQL statement is executed through the code, unless the code employs an explicit cursor. The developer does not explicitly declare the cursor, thus, known as implicit cursor.
E.g.:
In the following UPDATE statement, which gives everyone in the company a 20% raise, PL/SQL creates an implicit cursor to identify the set of rows in the table which would be affected.
UPDATE emp
SET salary = salary * 1.2;

Can you pass a parameter to a cursor? Explain with an explain
Parameterized cursor:
/*Create a table*/
create table Employee(
ID VARCHAR2(4 BYTE)NOT NULL,
First_Name VARCHAR2(10 BYTE)
);
/*Insert some data*/
Insert into Employee (ID, First_Name) values (01,’Harry’);
/*create cursor*/
declare
cursor c_emp(cin_No NUMBER)is select count(*) from employee where id=cin_No;
v_deptNo employee.id%type:=10;
v_countEmp NUMBER;
begin
open c_emp (v_deptNo);
fetch c_emp into v_countEmp;
close c_emp;
end;

/*Using cursor*/
Open c_emp (10);


What is a package cursor?
A Package that returns a Cursor type is a package cursor.
Eg:
Create or replace package pkg_Util is
    cursor c_emp is select * from employee;
    r_emp c_emp%ROWTYPE;
end;
/*Another package using this package*/
Create or replace package body pkg_aDifferentUtil is
    procedure p_printEmps is
    begin
        open pkg_Util.c_emp;
        loop
            fetch pkg_Util.c_emp into pkg_Util.r_emp;
            exit when pkg_Util.c_emp%NOTFOUND;
            DBMS_OUTPUT.put_line(pkg_Util.r_emp.first_Name);
        end loop;
        close pkg_Util.c_emp;
     end;
end;

Explain why cursor variables are easier to use than cursors.
Cursor variables are preferred over a cursor for following reasons:
A cursor variable is not tied to a specific query.
One can open a cursor variable for any query returning the right set of columns. Thus, more flexible than cursors.
A cursor variable can be passed as a parameter.
A cursor variable can refer to different work areas.


What is locking, advantages of locking and types of locking in oracle?
Locking is a mechanism to ensure data integrity while allowing maximum concurrent access to data. It is used to implement concurrency control when multiple users access table to manipulate its data at the same time.

Advantages of locking:
a.      Avoids deadlock conditions
b.      Avoids clashes in capturing the resources
Types of locks:
a.      Read Operations: Select
b.      Write Operations:  Insert, Update and Delete

What are transaction isolation levels supported by Oracle?
Oracle supports 3 transaction isolation levels:
a.      Read committed (default)
b.      Serializable transactions
c.       Read only

What is SQL*Loader?
SQL*Loader is a loader utility used for moving data from external files into the Oracle database in bulk. It is used for high performance data loads.

What is Program Global Area (PGA)?
The Program Global Area (PGA): stores data and control information for a server process in the memory. The PGA consists of a private SQL area and the session memory.

What is a shared pool?
The shared pool is a key component. The shared pool is like a buffer for SQL statements.  It is to store the SQL statements so that the identical SQL statements do not have to be parsed each time they're executed. 


38. What is snapshot in oracle?
A snapshot is a recent copy of a table from db or in some cases, a subset of rows/cols of a table. They are used to dynamically replicate the data between distributed databases. 

What is a synonym?
A synonym is an alternative name tables, views, sequences and other database objects.

What is a schema?
A schema is a collection of database objects. Schema objects are logical structures created by users to contain data. Schema objects include structures like tables, views, and indexes. 

What are Schema Objects?
Schema object is a logical data storage structure. Oracle stores a schema object logically within a tablespace of the database.

What is a sequence in oracle?
Is a column in a table that allows a faster retrieval of data from the table because this column contains data which uniquely identifies a row. It is the fastest way to fetch data through a select query. This column has constraints to achieve this ability. The constraints are put on this column so that the value corresponding to this column for any row cannot be left blank and also that the value is unique and not duplicated with any other value in that column for any row.

Difference between a hot backup and a cold backup
Cold backup: It is taken when the database is closed and not available to users.  All files of the database are copied (image copy).  The datafiles cannot be changed during the backup as they are locked, so the database remains in sync upon restore. 

Hot backup: While taking the backup, if the database remains open and available to users then this kind of back up is referred to as hot backup.  Image copy is made for all the files.  As, the database is in use the entire time, so there might be changes made when backup is taking place. These changes are available in log files so the database can be kept in sync

What are the purposes of Import and Export utilities?
Export and Import are the utilities provided by oracle in order to write data in a binary format from the db to OS files and to read them back.

These utilities are used:
·         To take backup/dump of data in OS files.
·         Restore the data from the binary files back to the database.
·         move data from one owner to another


Difference between ARCHIVELOG mode and NOARCHIVELOG mode
Archivelog mode is a mode in which backup is taken for all the transactions that takes place so as to recover the database at any point of time.
Noarichvelog   mode is in which the log files are not written. This mode has a disadvantage that the database cannot be recovered when required. It has an advantage over archivelog mode which is increase in performance.

What are the original Export and Import Utilities?
SQL*Loader, External Tables

What are data pump Export and Import Modes?
It is used for fast and bulk data movement within oracle databases. Data Pump utility is faster than the original import & export utilities.

What are SQLCODE and SQLERRM and why are they important for PL/SQL developers?
SQLCODE: It returns the error number for the last encountered error.
SQLERRM: It returns the actual error message of the last encountered error. 

Explain user defined exceptions in oracle.
A User-defined exception has to be defined by the programmer. User-defined exceptions are declared in the declaration section with their type as exception. They must be raised explicitly using RAISE statement, unlike pre-defined exceptions that are raised implicitly. RAISE statement can also be used to raise internal exceptions. 

Exception: 
DECLARE
userdefined  EXCEPTION;

BEGIN
<Condition on which exception is to be raised>
RAISE userdefined;

EXCEPTION
WHEN userdefined THEN
<task to perform when exception is raised>
 END;

Explain the concepts of Exception in Oracle. Explain its type.
Exception is the raised when an error occurs while program execution. As soon as the error occurs, the program execution stops and the control are then transferred to exception-handling part.
There are two types of exceptions:
1.      Predefined : These types of exceptions are raised whenever something occurs beyond oracle rules. E.g. Zero_Divide
2.      User defined: The ones that occur based on the condition specified by the user. They must be raised explicitly using RAISE statement, unlike pre-defined exceptions that are raised implicitly. 

How exceptions are raised in oracle?
There are four ways that you or the PL/SQL runtime engine can raise an exception:
·         Exceptions are raised automatically by the program.
·         The programmer raises a user defined exceptions. 
·          The programmer raises pre defined exceptions explicitly.

What is tkprof and how is it used?
tkprof  is used for diagnosing performance issues.  It formats a trace file into a more readable format for performance analysis. It is needed because trace file is a very complicated file to be read as it contains minute details of program execution.

What is Oracle Server Autotrace?
It is a utility that provides instant feedback on successful execution of any statement (select, update, insert, delete).  It is the most basic utility to test the performance issues.


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