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SQL Interview Questions

Last Updated: 26.04.2018

If you're looking for SQL Interview Questions for Experienced or Freshers, you are at right place. There are lot of opportunities from many reputed companies in the world. SQL database market continue even stronger by 2020 will be 70%. According to a research SQL by Microsoft grew at 10.32%, while Oracle grew 3.5%. In the upcoming era, competition will be more heated than is has been for years. So, You still have opportunity to move ahead in your career in SQL certification guide. Mindmajix offers Advanced SQL Interview Questions 2018 that helps you in cracking your interview & acquire dream career as SQL Developer.

SQL Interview Questions

Q. Difference between SQL Vs NoSQL ?

SQL Vs NoSQL
Feature SQL NoSQL
Type of Data Base Relational data base Non relational data base/Distributed data base
Standardardization Standard Query Language exist No proper standards defined
Reporting Tools Various tools available to analyse performance Unaivalability of tools to analyse data and performance
Development model Fine grained database model Architects can create new DB models
Price Expensive compared to NoSQL Low Cost - Mostly Open Source
Schema Predefined Schema available Unstructured data with dynamic schema
Database Examples PostGres, Sqlite, Oracle, etc., BigTable, Cassandra, MongoDB, etc.,
Type of Data storage Not suitable for hierarchial data storage Best suitable for hierarchial data.

Q. What is SQL?

SQL- A Structured Query Language, It is also pronounced as “SEQUEL” and it an Non-procedural Language which is used to operate all relational database. Used for Database communication. Its a standard language that can be used to perform the tasks like data retrieval, data update, insert or delete data from an database.

Features of SQL:

  1. Portability
  2. Client server architecture, 
  3. Dynamic data definition,
  4. Multiple views of data, 
  5. Complete database language, 
  6. Interactive, 
  7. High level,
  8. Structure and SQL standards.

Q. Difference between SQL & MYSQL ?

SQL is more natural than MYSQL. MySQL is a computer application. whose DBMS allows multiple users. It enables access to several database application and management system. SQL is more natural and standard language that is used with different applications alike. But, however no organization actually employs this standard language, rather every software firm follows its own kind of SQL version.

SQL Vs MySQL
SQL MYSQL
SQL stands for Structured Query Language MySQL is a RDMS (Relational Database Management System)
Allow for accessing and manipulating db's MySQL is a database management system, like SQL Server, Oracle,  Postgres, Informix etc
Basically works as the prompter to a DBMS It Facilitates multi-user access to a huge number of DBs
SQL codes & commands are used in various DBMS and RDBMS systems such as MySQL. MySQL has SQL at its core, and requires future upgrades mostly

Q. What is SQL Server?

SQL Server is Microsoft's relational database management system (RDBMS). End user cannot interact directly with database server. If we want to interact with SQL database server then we have to interact with SQL. 

Learn how to use SQL Server, from beginner basics to advanced techniques, with online video tutorials taught by industry experts. Enroll for Free SQL Server Training Demo!

Q. What are the Different SQL Servers Versions avialable in the market ?

SQL Servers Versions
SQL Server Code Name
SQL Server 2017 vNext
SQL Server 2016 Helsinki
SQL Server 2014 Hekaton
SQL Server 2012 Denali
SQL Server 2008 R2 Kilimanjaro
SQL Server 2008 Katmai
SQL Server 2005 Yukon
SQL Server 2000 Shiloh
SQL Server 7.0 Sphinx

Q. What are the different types of SQL’s statements?

1. DQL - Data Query Language ( or) Data Retrival Language 

  • SELECT Statement 

2. DML – Data Manipulation Language
    DML is used for manipulation of the data itself.

  • INSERT Statement
  • UPDATE Statement
  • DELETE Statement

3. DDL – Data Definition Language
    DDL is used to define the structure that holds the data. 

  • CREATE Statement
  • ALTER Statement
  • DROP Statement
  • RENAME Statement
  • TRUNCATE Statement

4. DCL – Data Control Language 
    DCL is used to control the visibility of data.

  • GRANT Statement
  • REVOKE Statement

5. TCL - Transaction Control Language

  • COMMIT Statement
  • ROLLBACK Statement
  • SAVEPOINT Statement

Q. What are various DDL commands in SQL? Give brief description of their purposes.

DDL Commands are used to define structure of the table

1. CREATE
It is used to create database objects like tables, views, synonyms, indexes.
Creating Table:
Syntax-
 Create table table_name(columname1 datatype(size), columname2 datatype(size),....);
 
 2. ALTER
It is used to change existing table structure.
Alter:: a) add
            b) modify
            c)drop
a) Add:
It is used to add columns into existing table
Syntax:
Alter table table_name add(columnname1 datatype(size), columname2 datatype(size),....);

b) Modify:
It is used to change column Datatype or datatype size only.
Syntax:
    Alter table table_name modify(columnname1 datatype(size), columnname2 datatype(size),....);

c) Drop:
It is used to drop columns from the table.
Method1:
If we want to drop single column at a time without using parentheses then we are using following syntax.
Syntax:
alter table   table_namedrop column   col_name1;  -- drop ONE column

Method2:
If we want to drop single or multiple columns at a time with using paranthesis then we are using following syntax.
Syntax:
alter table table_name drop(column_name_list);

Note:
In all databases we can’t drop all columns in the table.

3. DROP
It is used to remove database objects from database.
Syntax:
Drop object object_name; 
       (or)
Drop table table_name;  
       (or)
Drop view view_name;

4. RENAME
It is used to renaming a table.
Syntax:
Rename old table_name to new table_name;

Renaming a column:
Syntax:
Alter table table_name rename column old column_name to new column_name;

5. TRUNCATE
Oracle 7.0 introduced truncate table command it is used to delete all rows permanently from the table.
Syntax:
truncate table table_name;

Q. What are various DML commands in SQL? Give brief description of their purposes.

DML Commands are used to manipulate data within a table.
There are:: INSERT, UPDATE, DELETE

1INSERT −  It is used to insert data into in the table

Method1:
 Syntax
        Insert into table_name values(values1, value2, value3,……);

Method2:- Using Substitutional operator (&)
Synatx:
     Insert into table_name values(& columnname1, columnname2,.....);

Method3:- Skipping columns
Syntax:
       Insert into table_name(col1, col2,...) values(val1, val2, val3,...);

2. UPDATE - It is used to change data in a table.
Syntax:
      Update table_name set columnname=new value where columnname=old value;

Note: In all databases we can also use update statement for inserting data into particular shell.

3. DELETE - It is used to delete rows or particular rows from a table.
Syntax:
      Delete from table_name;
                        (or)
   Delete from tablename where condition;

Q. Difference Between Delete & Truncate?

SQL Delete Vs SQL Truncate
DELETE TRUNCATE
It is DML Command It is DDL Command
It is used to delete all the records row by row It is used to delete all the records at a time
By using delete command we can delete specific record By using truncate we cannot delete specific record                                                    
Where condition we can use with delete command Where condition will not work with truncate
Delete will work slow compare with truncate Truncate will work fast compare with delete
Delete will not reset auto generate id. Once when we delete all the records from the table. Truncate will reset auto generate id from starting number.

Q. About The SQL Buffer?

  • All Commands of SQL are Typed at the SQL prompt.
  • Only One SQL Statements is Managed in The SQL Buffer.
  • The Current SQL Statement Replaces the Previous SQL Statement in the Buffer.
  • The SQL Statement Can be Divided Into Different Lines Within The SQL Buffer.
  • Only One Line i.e., The Current Line Can be Active at a Time in the SQL Buffer.
  • At SQL Prompt, Editing is Possible Only in The Current SQL Buffer Line.
  • Every Statement of SQL should be terminated Using Semi Colon ”;”
  • One SQL Statement can Contain Only One Semo Colon.
  • To Run the Previous OR Current SQL Statement in the Buffer Type “/” at SQL Prompt.
  • To Open The SQL Editor Type “ED” at SQL Prompt.

Q. What are Important SQL Functions?

LOWER Function:(Column/Expression): 

  1. It Converts Alpha Character Values to Lower Case.
  2. The Return Value Has The Same Data Type as Argument CHAR Type (CHAR or VARCHAR2)

UPPER Function: 

  1. It Converts the Alpha Character Values to Upper Case.
  2. The Return Value Has The Same Data Type as Argument CHAR.

INITCAP Function:

  1. It Converts Alpha Character Values into Upper Case For The First Letter of Each Word, keeping all Other Letter in Lower Case.
  2. Words are Delimited by White Space or Characters That are Not Alphanumeric

LPAD Function:

  1. Pads The Character Value Right Justified to a Total Width of ‘n’ Character Positions.
  2. The Default Padding Character in Space.

RPAD Function:

  1. Pads the Character Value Left Justified to a Total Width of ‘n’ Character ositions.
  2. The Default Padding Character is Space.

LTRIM Function:

  1. It Enables to TRIM Heading Character From a Character String.
  2. All The Leftmost Character That Appear in The SET are Removed.

RTRIM Function:

  1. It Enables the Trimming of Trailing Character From a Character STRING.
  2. All the Right Most Characters That Appear in The Set are Removed.

TRIM Function:

  1. It Enables to TRIM Heading or Trailing Character or Both From a Character String.
  2. If LEADING is Specified Concentrates On Leading Characters.
  3. If TRAILING is Specified Concentrates on Trailing Characters.
  4. If BOTH OR None is Specified Concentrates Both on LEADING and TRAILING.
  5. Return the VARCHAR2 Type.

Advanced SQL Interview Questions & Answers

Q. How to Open SQL Server?

Goto -> Start -> All Programms -> Microsoft SQL Server 2008 R2 -> SQL Server management Studio.

Q.  What is SQL Injections? And How to Prevent SQL Injection Attacks?

It is a mechanism of getting secure data from database.

SQL Injection Attacks::

  • By providing proper validations for input fields.
  • By using parameterised queries.
  • By using stored procedures
  • By using frequent code reviews
  • We must not display database error messages in frontend
  • SQL injection is a code injection technique, used to attack data-driven applications.

Q. Difference Between Scalar Valued Functions & Table Valued Functions in SQL?

SQL Scalar Valued Functions Vs SQL Table Valued Functions
Scalar Valued Functions Table Valued Functions
It will process on single row ata time & return only one value of any database It will process on multiple rows at a time & return multiple rows (or) single row from table
The return type of scalar valued function is datatype The return type of table valued function is table     
Scalar valued function will have as begin block end Table valued function will not have as begin end

Syntax to call Scalar Valued Functions is::

SELECT dbo. funname(values);

Syntax to call Table Valued Functions is::

SELECT  * FROM dbo.funname(values);

Q. Difference Between Stored Procedure & Functions?

Stored Procedure Functions
It is a set of pre-compiled SQL Statements which will gets executed when we call it It will take input from user and return only one value of any data type              
Compile only one time Compile every time
Stored Procedure will have execution plan Function will not have execution plan
Support DML Commands Not supported DML Commands
Support TCL Commands Not supported TCL Commands
It is may or may not have input parameter Function must have at least one input parameter
It is accept both input and output parameters Doesn’t have output parameters
We call call stored procedure in another stored procedure We can call function in another function
It is support Exception Handling It is doesn’t support Exception Handling
We can call function in stored Procedure We can’t call stored procedure in function

Q. Write a Query to view the indexes that are applied on the table?

 stored procedure_helpindex  table_name

Q. Difference Between Long & Lob Datatypes?

Long Lob
It stores upto 2GB Data It stores upto 4 GB Data               
A table can contain only one long column A table can contain more than Lob column            
Subquery cannot select a Long datatype column Subquery can select Lob Column

Q. Difference Between (Null Value Function) nvl() & Coalesce()

  • Nvl is an oracle function whereas Coalesce is an ANSI Function and also coalesce performance is very high as compare to NVL Function.
  • NVL Function internally uses implicit conversions i.e NVL Function returns a value if the exp1, exp2 is not belong to same datatype also if exp2 automatically converted into exp1 where as in coalesce function exp1, exp2 must belongs to same datatype.

Examples1::
         SELECT nvl(‘a’, sysdate) FROM dual;
Output::
         a

Examples2::
         SELECT Coalesce(‘a’, sysdate) FROM dual;
Error: inconsistent datatypes: expected CHAR got DATE

Q. Difference Between Views & Materialized Views?

Views Materialized Views
View does not store data Materialized view stores data
Security purpose Improved performance purpose
When we ar4e dropping base table then view can’t be accessible When we are dropping base table also materialized view can be accessible
Through the view we can perform DML Operation                               We can’t perform DML operation

                                                Checkout SQL Tutorials

Q. What is Tuple?

Tuples are the members of a relation. An entity type having attributes can be represented by set of these attributes called tuple.

Q. What is Query & Query Language?

  • A query is a statement requesting the retrieval of information. 
  • The portion of dimly that involves information retrieval is called a query language.

Q. What are the different aggregate functions in SQL?

AVG(), MIN(), MAX(), SUM(), COUNT()

Q.What is data independence?

A database system keeps data separate from software data structure.

Q. What is data integrity?

Data must satisfy the integrity constraints of the system for data Quality.

Q. What is Dead locking?

It is the situation where two transactions are waiting for other to release a lock on an item.

Q. What is decryption?

Taking encoded text and converting it into text that you are able to read.

Q. What is two phase locking?

It is a most common mechanism that is used control currency in two phases for achieving the serializability. The two phases are growing and shrinking.

A transaction acquires locks on data items it will need to complete the transaction. This is called growing process. A transaction may obtain lock but may not release any lock.
One lock is released no other lock may be acquired this is called shrinking process. A transaction may release locks but may not obtain any new locks.

Q. What is projection?

The Projection of a relation is defined as projection of all its tuples over a set of attributes. It yields vertical subset of the relation. The projection operation is used to view the number of attributes in the resultant relation or to reorder attributes.

Q. What is Encryption?

Encryption is the coding or scrambling of data so that humans can not read them directly.

Q. What is cardinality?

The number of instances of each entity involved in an instance of a relation of a relationship describe how often an entity can participate in relation ship. (1:1, 1:many, many:many).

Q. What is Transaction Control?

Oracle Server Ensures Data Consistency Based Upon Transaction.
Transactions Consist of DML Statements That Make Up One Consistent Change To The Data,

Q. What are the Transaction Start & End Cases?

  • A Transaction Begins When The First Executable SQL Statement is Encountered.
  • The Transaction Terminates When The Following Specifications Occur.
  1. A COMMIT OR ROLLBACK is Issued
  2. A DDL Statement Issued.
  3. A DCL Statement Issued.
  • The Usr Exists The SQL * Plus
  • Failure of Machine OR System Crashes.
  • A DDL Statement OR A DCL Statement is Automatically Committed And Hence Implicitly Ends A Transaction.

Q. GRANT Command?

Syntax::
SQL> GRANT< Privilage Name1>, ,
           ON
           TO ;
GRANT Command is Used When We Want The Database To Be Shared With Other Users.

The Other Users Are GRANTED With Certain Type of RIGHTS.
GRANT Command Can Be issued Not Only on TABLE OBJECT, But Also on VIEWS, SYNONYMS, INDEXES, SEQUENCES Etc.

SQL> GRANT SELECT
          ON EMP
          TO ENDUSERS;
SQL> GRANT INSERT, SELECT, DELET
          ON EMP
          TO OPERATORS;
SQL> GRANT INSERT (Empno, Ename, Job)
          ON Emp
          To EndUsers;

Q. REVOKE Command?

Syntax::
SQL> REVOKE< Privilage Name1>, ,
          ON
           FROM;
REVOKE Command is Used When We Want One Database To Stop Sharing The Information With Other Users.
Revoke Privileges is Assigned Not Only On TABLE Object, But Also on VIEWS, SYNONYMS, INDEXES Etc.
SQL> REVOKE INSERT, DELETE
          ON EMP
          FROM Operators;

Q. Connecting to Oracle OR SQL * Plus?

Double Click the SQL*Plus ShortCut on the Desktop.

Start -> Run -> Type SQLPlus OR SQLPlusW in Open Box and Click OK.

Start -> Programs -> Oracle -> Application Development -> SQL*Plus

In the Login Box OR Login Prompt Type the User Name and Password as Supplied by the Administrator.

The Host String is Optional and is provided by the Administrator.

Q. About PL/SQL Tables?

  • Objects of Type “TABLE” Are Called PL/SQL Tables.
  • They Are Modeled As Database Tables, But Are Not Same.
  • PL/SQL TABLES Use A “PRIMARY KEY” To Give Array Like Access To Rows.
  • PL/SQL Tables Are Very Dynamic in Operation, Giving The Simulation To Pointers in ‘C’ Language.
  • They Help in Integrating The Cursors For Dynamic Management of Records At Run Time.
  • They Make Runtime Management of Result Sets Very Convenient.

                                Frequently Asked Oracle PL SQL Interview Questions & Answers

Q. Difference Between SQL and PL/SQL?

SQL PL/SQL
It’s complete name is structured query language It’s complete name is procedural Language / Structured Query Language
It doesn’t have the any facility of branching or looping It has the complete  facility of branching or looping
In SQL, only one statement can be sent to Oracle Engine. It increase the execution time In PL/SQL, a complete block of statements can be sent to Oracle engine at a time, reducing traffic
In SQL, the use of variables is not possible In PL/SQL, the results of the statements can be stored in variables and can be used further as per the requirement
It doesn’t have the capacity for procedural language It is fully support procedural language
In SQL, there is no facility of error management. In case of error condition, It is the Oracle Engine that tracks it.                     In PL/SQL, the results of the statements can be stored in variables and can be used further as per the requirement    

Q. What is the Temporary Tables?

Syntax: CREATE TABLE  #TempTab()
A Temporary Table or Temp-Table is created on disk in the tempDB system database. The name of this Temp-Table is suffixed with a session-specific ID so that it can be differentiated with other similar named tables created in other sessions. The name is limited 116 chars.

Example:

Here is an example showing you the usage of a temporary table.
mysql> CREATE TEMPORARY TABLE SALESSUMMARY (
   -> product_name VARCHAR(50) NOT NULL
   -> , total_sales DECIMAL(12,2) NOT NULL DEFAULT 0.00
   -> , avg_unit_price DECIMAL(7,2) NOT NULL DEFAULT 0.00
   -> , total_units_sold INT UNSIGNED NOT NULL DEFAULT 0
);
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO SALESSUMMARY
   -> (product_name, total_sales, avg_unit_price, total_units_sold)
   -> VALUES
   -> ('cucumber', 100.25, 90, 2);

mysql> SELECT * FROM SALESSUMMARY;
+--------------+-------------+----------------+------------------+
| product_name | total_sales | avg_unit_price | total_units_sold |
+--------------+-------------+----------------+------------------+
| cucumber     |      100.25 |          90.00 |                2 |
+--------------+-------------+----------------+------------------+
1 row in set (0.00 sec)

Q. What are the types of SQL operators?

SQL Arithmetic Operators

Operator Description
+ Add
- Subtract
* Multiply
/ Divide
% Modulo

SQL Bitwise Operators

Operator Description
& Bitwise AND
| Bitwise OR
^ Bitwise exclusive OR

SQL Compound Operators

Operator Description
+= Add equals
-= Subtract equals
*= Multiply equals
/= Divide equals
%= Modulo equals
&= Bitwise AND equals
^-= Bitwise exclusive equals
|*= Bitwise OR equals

SQL Comparison Operators

Operator Description
= Equal to
> Greater than
< Less than
>= Greater than or equal to
<= Less than or equal to
<> Not equal to

SQL Logical Operators

Operator Description
ALL TRUE if all of the subquery values meet the condition
AND TRUE if all the conditions separated by AND is TRUE
ANY TRUE if any of the subquery values meet the condition
BETWEEN TRUE if the operand is within the range of comparisons
EXISTS TRUE if the subquery returns one or more records
IN TRUE if the operand is equal to one of a list of expressions
LIKE TRUE if the operand matches a pattern
NOT Displays a record if the condition(s) is NOT TRUE
OR TRUE if any of the conditions separated by OR is TRUE
SOME TRUE if any of the subquery values meet the condition

Q. What is a CURSOR?

CURSOR is a Handle, OR Pointer To The CONTEXT AREA

Q. What is The CURSOR Usage?

Using a CURSOR, The PL/SQL program can control the CONTEXT AREA, As the SQL Statement is being processed.

Q. What are the CURSOR Features?

  • CURSOR Allows to FETCH and process Rows returned by a SELECT statement, One Row at a time.
  • A CURSOR is named, such that it can be referenced by the PL/SQL programmer dynamically at run time.

Q. What are the Cursor Types?

CURSORS are broadly recognized as 2 types

  1. Implicit Cursors
  2. Explicit Cursors

Implicit Cursors:

  • It is a cursor that is automatically declared by oracle every time an SQL statement is executed.
  • The programmer cannot control OR process the information in an Implicit Cursor.

Explicit Cursors:

  • It is a Cursor that is defined by the programmer within the program for any Query that returns more than one row of data.
  • This Cursor is declared within the PL/SQL block, and allows sequential process of ach row of the returned data from database.

Q. What are the Different Types of Constraints?

  1. Null Constraint
  2. Not Null Constarint
  3. Primary Key Constraint
  4. Unique Key Constraint
  5. Foreign Key Constraint
  6. Composite Primary Key Constraint
  7. Default Constraint
  8. Check Constraint

Q. Explain About Different Types of Constraints?

1. Null Constraint - It allows Null Values

Syntax: create table table_name(columnname datatype, columnname datatype null);

2. NOT NULL Constraint - A NOT NULL Constraint Prohibits a Column From Containing NULL Values.

  • NOT NULL Should Be defined Ony At COLUMN Level.
  • The Default Constraint if Not Specified is NULL Constraint.
  • To Satisfy the Rule, Every Row in The Table Must Contain a Value For The Column.

Syntax:


CREATE Table Table_Name
(
Column_Name1(Width) NOT NULL,
Column_Name2(Width)
CONSTRAINT ConsName NOT NULL,
Column_NameN(Width)
);

3.UNIQUE Constraint - The UNIQUE Constraint Designates a Column A s a UNIQUE Key.

  • A Composite UNIQUE Key Designates a Combination of Column As The UNIQUE Key.
  • A Composite UNIQUE Key is Always Declared At The Table Level.
  • To Satisfy a UNIQUE Constraint, No Two Rows in Tha Table Can Have The Same Value For The UNIQUE Key.
  • UNIQUE Key Made Up of a Single Column Can Constraint NULL Values
  • Oracle Creates An Index Implicitly on The UNIQUE Key Cokumn.

Synatx:

Create Table
(
Column_Name1(Width) UNIQUE,
Column_Name2(Width)
CONSTRAINT ConsName UNIQUE,
Column_NameN(Width)
);

4. PRIMARY KEY Constraint - A PRIMARY KEY Constraint Designates a Column A sThe PRIMARY KEY of a TABLE or VIEW

  • A COMPOSITE PRIMARY KEY Designates a Combination of Columns As The PRIMARY KEY.
  • When The Constraint is Declared At Column Level Only PRIMARY KEY Keyword is Enough.
  • A Composite PRIMARY KEY is Always Defined At Table Level Only.
  • A PRIMARY KEY Constraint Combines a NOT NULL and UNIQUE Constraint in One Declaration.

Synatx:

CREATE Table< Table_Name>
(
Column_Name1(Width)
CONSTRAINT ColNamePK PRIMARY KEY,
Column_Name2(Width),
Column_NameN(Width)
);

5. FOREIGN KEY Constraint - It is Also Called As REFERENTIAL INTEGRITY CONSTRAINT. It Designates a Column as FOREIGN KEY And Establishes a RELATION Between The FOREIGN KEY And a Specified PRIMARY OR UNIQUE KEY. A COMPOSITE FOREIGN KEY Designates a Combination of Column As The FOREIGN KEY.

  • The TABLE or View Containing The FOREIGN KEY is called the Child Object.
  • The TABLE or View Containing The REFERENCED KEY is called the Parent Object.
  • The FOREIGN KEY And The REFERENCE KEY Can Be in The Same TABLE or VIEW.
  • The Corresponding Column or Columns of the FOREIGN KEY And The REFERENCE KEY Must Match in ORDER and DATA TYPE.

A FOREIGN KEY CONSTRAINT Can Be Defined on a Single Key Column Either Inline or Out of Line
A COMPOSITE FOREIGN KEY on Attributes  Should Be Declared at Table LEVEL or Out of Line Style. We Can Designate The Same Column or Combination of Columns as Both a FOREIGN KEY and a PRIMARY or UNIQUE KEY. A COMPOSITE FOREIGN KEY CONSTRAINT, Must Refer To a COMPOSITE UNIQUE KEY or a COMPOSITE PRIMARY KEY in the PARENT TABLE or VIEW.

Syntax:

Create table table_name(columnname datatype foreign key references primary key table_name(primarykey column name);

6. COMPOSITE PRIMARY KEY Constraint - Applying PRIMARY KEY Constraint for the combination of columns is called as COMPOSITE PRIMARY KEY Constraints.

  • It will not allow duplicate values.
  • It will not allow null values

We cannot apply more than  one COMPOSITE PRIMARY KEY Constraints on a single table.

Syntax:

Create table table_name(colname1 datatype, colname2 datatype PRIMARY KEY(col1, col2));

7. CHECK Constraint -Check constraints are used to ensure the validity of data in a database and to provide data integrity.

Syntax:
Create table table_name(columnname datatype check(condition));

8. DEFAULT Constraint:

It is used to insert default value instead of null values.

Syntax:

Create table table_name(columnname datatype, default value);

Q. %Found

  • This attribute returns boolean value either true or false.
  • This attribute returns true when fetch statement returns atleast one records.

Syntax:

cursorname%found
SQL>declare
Cursor c1 is select * from emp
Where ename =’&ename’;
i emp% rowtype;
begin
open c1;
fetch c1 into i;
If c1%found then
dbms_outpit.put_line(your employee exists’||’ ‘||i.ename|| ‘ ‘||i.sal);
else if c1%not found then
dbms_output.put_line(‘your employee does not exists’);
end if;
close c1;
end;
/

Output: enter value for ename:murali
Employee doe snot exists

Output: enter value for ename:KING
Employee exists KING 7400

Q. Explain Eliminating Explicit Cursor Life Cycle (or) Cursor FOR Loops?

Using cursor for loop we are eliminating explicit cursor life cycle i.e whenever we are using cursor for loop no need to use open, fetch, close statement explicitly i.e when we are using cursor for loop oracle server only internally automatically open the cursor, and then fetch data from the cursor and close the cursor.

Syntax:For index

varname in cursorname
Loop
stmts;
end loop;

Note:
In cursor for loop index variable internally behaves like a record type variable. (%row type)

Q. What are the Cursor Attributes?

Attribute Name Return Value Condition
%found

True

-------------

False         

If fetch statement return at least one row

-------------------------------------------------

If fetch statement doesn't returns any row                

%notfound

True

------------

False

If fetch statement doesn't returns any row

--------------------------------------------------

 If fetch statement return at least one row

%isopen

True

----------

False

If cursor is already opened

--------------------------------------------------

If cursor is not opened

%rowcount

Number

If counts number of records number fetches from the cursor

Q. What is Autonomous Transaction?

Autonomous transactions are independent transaction used in anonymous blocks, procedures, functions, triggers.
Generally we are defining autonomous transaction is child procedure.
Whenever we are calling autonomous procedure in main transaction and also main transaction TCL commands never affected on autonomous TCL commands procedure, because these are independence procedure.
If we want to procedure autonomous then we are using autonomous transaction pragma, commit i.e in declare section of the procedure we are defining autonomous transaction pragma and also we must use commit in procedure coding.

Q. What is Out Mode?

We can also use out mode parameter i function, but these functions are not allowed to execute by using select statement. If we want to return more no.of values from a function then only we are allowed to use out parameter. Here also out parameter behaves like a uninitialized variables.

Q. What is SQL Loader?

SQL Loader is an utility program which is used to transfer data from flat into oracle database. SQL Loader always executes control file based on the type of flat file we are creating control file and then submit conrol file to SQL loader then only SQL loader transfer file into flat file into oracle Data Base during this file some other files also created.

  1. Logfile
  2. Badfile
  3. Discardfile

Q. SQL%bulk_rowcount

Oracle introduced sql%bulk_rowcount attribute which is used to count affected number of rows within each group in bulk bind process. (forall statements).
Syntax: sql%bulk_rowcount(index varname);

Q. Authid current_user

When a procedure have a authid current_user clause then those procedures are allowed to execute only owner of the procedure.
These procedures are not allowed to executes by another users if any user givings permission also. Generally whenever  we are reading data from table and performs some DML operations then only data security principles of view developers uses this clause in procedures.
This clause are used in procedures specification only.

Synatx:

Create or replace procedure procedurename(formal parameters)
Authid current_user
is/as
-----------
Begin
----------
[exeception]
------------
End[procedurename];

Q. What is Row-Level-Attribute?

In this method a single variables can represent all different datatype into single unit. This variable is also called as record type variable.
Row Level Attribute are represented by using %rowtype.S

yntax:
variable_name table_name%rowtype;

Q. What are types of Blocks in PL/SQL?

PL/SQL having 2 types of blocks
1. Anonymous Block
2. Nammed Block

Anonymous Block Nammed Block
This block doesnot have a name This block having a name
These blocks are not stored in oracle database These blocks are automatically permanently stored in Database
Thess blocks are not allowed to call in client application These blocks are allowed to call in client application

Q. Write a PL/SQL cursor program which is used to display total salary from emp table without using sum() function by using cursor for loop?

sql> declare
        cursor c1 is select*from emp
        n number(10):=0;
        begin
        for i in c1
        loop
        n:=n+i.sal;
        end loop;
        dbms_output.put_line (‘total salary is: ‘||’ ‘||n);
        end;
         /
Output: total salary is: 42075

Q. What is Normalization?

Normalization is a scientific process which is called to decomposing a table into number of tables. This process automatically reduces duplicate data and also automatically avoids insertion, updation, deletion problems.
In design phase of SDLC database designers designs LOGICAL MODEL of the database in this logical model only database designers uses normalization process by using normal forms.

Q. What is Super Key?

A columns or a combination of columns which uniquely identifying a record in a table is called a Super Key.

Q. What is Candidate Key?

A minimal super key uniquely identifying a record a table is called candidate key 
(or) 
A super key which is a subset of another super key then those super keys are not a candidate key.

Q. What is Bad File?

This file extension is .bad
Bad file stores rejected records based on

  1. Data type mismatch
  2. Business rule violation

Bad file is automatically created as same name as Flat file, we can also create Bad file explicitally by using bad file clause within control file.

Q. What is Discard File?

This file extension is .dsc
Discards file we must specify within control file by using discard file clause.
Discard file also stores rejected record based on when clause condition within control file. This condition must be satisfied into table table_name clause.

Q. What is Autoincrement?

In all databases generating primary key value automatically is called auto increment concept. In Oracle we are implementing auto increment concept by using row level triggers, sequences. i.e here we creating sequence in sql and use this sequence in PL/SQL row level trigger.
SQL> create table test (sno number(10), primary key, name varchar2(10));

Q. What is Dynamic SQL?

It is the combination of SQL, PL/SQL i.e SQL statements are executed dynamically with PL/SQL block using execute immediate clause.
Generally in PL/SQL block we are not allow to use DDL, DCL statements using Dynamic SQL DDL, DCL statement within PL/SQL block.

Syntax: 

begin
execute immediate ‘sql statement’
end;
/

Q. Write a dynamic SQL program to display number of records from emp table?

SQL> declare
           z number(10);
           begin
           execute immediate ‘select count * FROM emp’
           into z;
           dbms_output.put_line(z);
            end;
            /

Q. Write a dynamic SQL program for passing department number 20 retrieve deptnames, Loc from dept table?

SQL> declare
          v_deptno number(10):=20;
          v_dname varchar2(10);
          v_loc varchar2(10);
          begin
          execute immediate ‘select dname, loc FROM dept where deptno=1’ into v_dname, v_loc using            v_deptno;
          dbms_output.put_line(v_dname ||’ ‘|| v_loc);
          end;
          /

 

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