createing view

          1Why Use Views
          ? to restrict data access
          ? to make complex query easy
          ? to provide data independence
          ? to provide defferent view of the same data
          2 Creating a View
          ? 1)create [or replace] [force|noforce] view view
          ? as subquery
          ? force : create view wether the referenced object existed or not
          ?
          ? desc view_name;
          ?2)create a view by using column aliases in the subquery
          ? create view salv50
          ? as select employee_idIO_NUMBER,last_name NAME,slaary*12 ANN_SALARY
          ? from employees
          ? where department_id=50;
          3 Modigy a View
          ? 1) Modigy the emplvu80 view by using create or replace view clause.Add an alias for each

          column name;
          ?? create or replace view empvu80
          ?? (id_number,name,sal,department_id)
          ?? as select employee_id,first_name||" "||last_name,salary.department_id
          ?? from employees
          ?? where department_id=80;
          ?? column aliases in the create view clause are listed in the same order as the columns in

          the subquery
          ?? note : alter view_name is not a valid command.
          4 Create a Complex View
          ? Create a complex view that contains group functions to display values from two tables
          ? create view dept_sum_vu
          ?? (name,minsal,maxsal,avgsal)
          ? as
          ?? select d.department_name,min(e.salary),max(e.salary),avg(e.salary)
          ?? from employees e,departments d
          ?? where e.department_id=d.department_id
          ?? group by d.department_name;
          5 Rules for performs DML operaton on a view
          ? 1) You can perform DML operation on simple views
          ? 2) You can not romove a row if the view contains the following:
          ??? --group functions
          ??? --a group by clause
          ??? --the distince keyword
          ??? -- rownum keyword
          ??? -- column defined by expressions
          6 Using the with check option Clause
          ? 1) you can ensure that dml operatons performed on the view stay within the domain of the

          view by using the with check option clause.
          ? creaate view test1
          ? as
          ? select * from emp where qty>10;
          ? with check option;
          ? update testview1 set qty=10
          ? where ster_id=6830;
          ? --when you doing the following update operation
          ? update testview1 set qty=5 where id=10;
          ? -- an error will report
          ? --you violate the where clause
          ? 2)Any attempt to change the department number for any row in the view fails because it

          violates the with check option constraint
          ?? create or replace view empvu20
          ?? as
          ?? select * where department_id=20
          ?? with check option constriant empvu20_ck;
          7 Denying DML Operations
          ? 1 You can ensure that no dml operations occur by adding the with read only option to your

          view definition.
          ? 2)Any attempt to a DML on any row in the view resuls in an oralce server error.
          8 remove veiw
          ? drop view_name
          9 inline view
          ? 1) an inline view is a subquery with an alias that you can use within a sql statement.
          ? 2) a named subquery in the from clause of the main query is an exqmple of an inline view
          ? 3) an inline view is not a schema object.
          10 Top-N Analysis
          ?1)Top_N querise ask for the n largest or smallest values of a column.
          ?2)Both largest values and smallest values sets considered Top-N queries
          ? select * from (select ster_id,qty from sales);
          ?example
          ? To display the top three earner names and salaries from the employees
          ? select rownum as rank,last_name,salary
          ? from (select last_anme,slary from employee
          ??????? order by slary desc)
          ? where rownum<=3;
          ?

          posted on 2006-09-27 18:30 康文 閱讀(276) 評論(0)  編輯  收藏 所屬分類: 數據庫

          <2006年9月>
          272829303112
          3456789
          10111213141516
          17181920212223
          24252627282930
          1234567

          導航

          統計

          常用鏈接

          留言簿(1)

          隨筆分類

          隨筆檔案

          文章檔案

          搜索

          最新評論

          閱讀排行榜

          評論排行榜

          主站蜘蛛池模板: 西畴县| 温州市| 老河口市| 泗洪县| 承德市| 邵东县| 富蕴县| 财经| 开远市| 迁安市| 林口县| 同心县| 东至县| 太康县| 宜城市| 神木县| 武安市| 桦甸市| 巴林右旗| 湛江市| 泌阳县| 东平县| 彭泽县| 中牟县| 新昌县| 高雄市| 巴中市| 大余县| 承德县| 水城县| 延津县| 托里县| 上林县| 三都| 镇远县| 西盟| 漳浦县| 安龙县| 呼和浩特市| 枞阳县| 方城县|