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    Views Views Presentation Transcript

    • VIEWS Chapter 14
    • What is a view?
      • A VIRTUAL table in the database whose contents are defined by a query
      • SQL query permanently stored in the database with a name
      • To a user a view appears the same as a table
      • But unlike a real table,records of the view are not stored in the database
      • The data in the view comes from the source tables.
    • Creating a view
      • Create view V_emp_dept as select ename , dname from emp,dept where emp.deptno=dept.deptno;
      • Once the view is defined it can be used in select statement like a table
      • Select * from V_emp_dept ;
    • Advantages of Views
      • Security:Users to be given permission to access db through views
      • Query simplicity
      • Structural simplicity
      • Dynamic nature:definition of view unchanged when table updated
      • Data Integrity maintained
    • Disadvantages of Views
      • Performance issues
      • Update restrictions:not always possible to update views
    • Types of views
      • Horizontal view
      • Vertical view
      • Row/Column subset view
      • Grouped view
      • Joined view
    • Horizontal view
      • Create view hvdept10 as select * from emp
      • where deptno=10;
      • Access to Only selected rows
      • Slices source table horizontally to create view
      • All columns of source table are part of the view
    • Vertical view
      • Create view vvemp
      • as select ename,sal,comm from emp;
      • Access to Only selected columns
      • Slices source table vertically to create view
      • All rows of source table are part of the view
    • Row/Column subset view
      • Create view rcemp as select ename,sal,comm from emp
      • where deptno=10;
      • Access to selected columns and rows
      • Only the columns named in select list and rows that meet search condition form part of the view
    • Grouped View
      • Create view gvemp(department,no_emp,total_sal, avg_sal,max_sal,min_sal)
      • as select deptno,count(*),sum(sal),
      • avg(sal),max(sal),min(sal)
      • from emp
      • group by deptno;
      • Select * from gvemp;
    • Grouped View(contd.)
      • Select query has a group by clause
      • Produce one row for each group
      • Definition always includes a list of column names
      • Do not have one to one correspondence with source table rows
      • Summary of source table/tables
      • Cannot be updated
    • Joined View
      • Create view V_emp_dept as select ename , dname from emp,dept where emp.deptno=dept.deptno;
      • Draws data from two or more tables
      • For the user ,querying becomes easy
    • Updating Views
      • Whenever updates are made through the view the source table data is also updated
      • Views may be updateable or non updateable
      • Generally views made with simple queries are updateable
    • Restrictions on updating views
      • No distinct used in select
      • From clause must have one source table
      • Each select item a simple column name
      • Where clause should not have a sub query
      • No group by or having clause
    • With Check Option
      • To prevent updation of rows which are not visible through the view WITH CHECK OPTION is used
      • SQL automatically checks each Insert and update operation for the view to make sure, resulting row meets search condition in view definition
    • With Check Option(Example)
      • create view rcemp
      • as select ename,sal,comm
      • from emp where sal>1000
      • with check option;
      • View created
      • update rcemp set sal=800 where ename='ALLEN';
      • ORA-01402: view WITH CHECK OPTION where-clause violation
    • Dropping a view
      • Drop view <view_name>;
      • Only view dropped not the source tables
    • Materialized views
      • When the query defining a view is complex the dbms materializes the view
      • Saves results of query temporarily in a table
      • Discards the table when query is over
      • Materialized view -query run once,data saved permanently,with every update to source table corresponding changes to view
    • THANKS