Difference between Subquery and Joins?

Sub query is also query which returns one or more values,we
can retrieve the data based on the condition...

Join is used to join the two tables and used to retrieve the
data from one or more tables.

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What is Primary and Foreign key in databases?

Primary key:

   Each table can have one primary key, which uniquely identifies each row in a table and ensures that no duplicate rows exist.
 

   Foreign Key:

   On whichever column you put FOREIGN KEY constraint then the values in that column must refer to existing values in the other table. A foreign key column can refer to primary key or unique key column of other tables.

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Explain about the following things in Databases?

1. What is foreign key and Primary key , how do you differentiate?
2. What is Index in the table?
3. How do we differentiate producer and function in Database?
4. What is trigger in Databases?

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what is view in SQL Server?

A view is a virtual table that consists of columns from one or more tables. Though it is similar to a table, it is stored in the database. It is a query stored as an object. Hence, a view is an object that derives its data from one or more tables. These tables are referred to as base or underlying tables


syntax:
CREATE VIEW vwCustomer
AS
SELECT CustomerId, Company Name, Phone
FROM Customers


For more information, see the below link..



http://www.sql-server-performance.com/articles/dev/views_in_sql_server_p1.aspx

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How to take backup in oracle?

 For Exporting the Database :
exp file=RPM62B13.dmp userid=devusr/Usrdev1234@RPM62B13.symphonysv.com fully=y
log=AgileExpBackup.log

For Exporting the tables :
exp  file=empdept.expdat  userid=devusr/Usrdev1234@RPM62B13.symphonysv.com tables=(EMP,DEPT) log=empdept.log

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Drop and truncate in Database?

drop table tablename;
  It will  delete per-mentally the table and data.
 truncate table tablename;
  it will delete the data from the table.

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How to copy the table in oracle?

•  Copy the table structure with data

        create table emp_copy as
          select *
            from Employee;

•   Copy the table structure with out data  

            CREATE TABLE Table2 AS 
               SELECT * FROM Table1 WHERE 1=0;
       Note: if condition is not validating then it wont copy the data. 

•  Copying selected columns from another table

                   CREATE TABLE newTable
                        AS (SELECT empno, ename
                                    FROM emp);

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constraints in Oracle

They are five integrity constraints are available in Oracle.

1. Not Null
2. Primary key
3. Foreign key
4. Check
5. unique    

For more information , Refer the following URLs
 http://www.oracle-dba-online.com/sql/integrity_constraints.htm

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