CSED421 Database Systems Lab View

Page 2  Virtual or logical table which is defined as SELECT query with joins  Act as table but not real data, only query  Simply, predefined query which frequently will be used  In select query  In ‘From’ clause subquery is not allowed  In ‘Where’ clause subquery is allowed  Other things are same as real table manipulation What is view?

Page 3 What is view?

Page 4 Why use view?  Select ShipperName from Shipper s where 5< (select count(*) as total from Orders o, Customers c where s.ShipperID=o.ShipperID and o.CustomerID=c.CustomerID and Country=‘Germany’) Order by ShipperName  Select * from v where v.total>5 Define this as view v

Page 5  Simplify complex queries  Reuse or represent same queries(view) as different alias  Limit data access to specific users  Reduce the network traffic Why use view?

Page 6  Creating view  Create View view_name (column name) As Select_statement  Modifying view  Create or Replace VIEW view_name (column name) As Select_statement  Dropping view  DROP VIEW view_name  Show full tables  Desc view_name  Select * from view_name Syntax of view

Page 7  Find the number of order from German  Create View german_customer As Select * from customers Where country=‘Germany’  Select sum(orderQuantity) from german_customer  Select sum(orderQuantity) from (Select * from customers where country=‘Germany’) as german_customer Example of view

Page 8  It is possible to update data in the underlying table through the view  One-to-one relation between the rows in view and underlying table  SELECT statement must follow several rules  Only refer to one database table  Not use GROUP BY or HAVING clause  Not use DISTINCT  NOT contain any expression(aggregates, functions, computed columns ⋯ )  Update v set col1=0 where col1<0 Updatable view

Page 9  With Check Option clause can be given for an updatable view to prevent insert or update to rows except those for which the WHERE clause in the select_statement is true  Example  Create View emp80 As Select employee_id, last_name, department_id From employees Where department_id=80 With Check Option  Update emp80 Set department_id=10 Where employee_id=145 → error occurred With Check Option Clause

Page 10   위 사이트에서 코드 실행 후 SQL 문 올려주세요. Practice Info

Page 11  1. 각 Order 에 대한 Customer 의 이름을 보여주는 view 를 생성 하시오.  View name : orderName_vu Practice

Page 12  2. 각 order 의 판매 가격을 보여 주는 view 를 만드시오.  viewname : orderPrice_vu  판매 가격 = product price * quantity Practice

Page 13  3. Order 의 판매 가격이 가장 높은 customer 의 이름을 찾으시오.  Hint: practice 1,2 의 view 를 이용 Practice