1. Views, Algebra Temporary Tables

2. Definition of a view A view is a virtual table which does not physically hold data but instead acts like a window into one or more physical tables (known as base tables). To the end user, a view looks like a table and can generally be treated as a table. CREATE VIEW [ ( ) ] AS SELECT [ (WITH CHECK OPTION); ]

3. Uses of views Save the user time if a complex query needs to be re-executed. Restrict data that user can access. Such restrictions might occur : - –on records, –on attributes (columns) or –on both (security feature).

4. Notes The AS SELECT clause can use all variations of the standard SELECT statement except cannot use UNION or ORDER BY. This means that a view can be based on a combination of tables and / or views. The 'data' in views can be listed by using a SELECT statement. Mathematical functions can be used in creation of views.

5. Non-base fields Derived fields Sum Average Count Summing group fields

6. View from Northwind Create view ProductIncome as Select ProductID, cast(sum((unitPrice*Quantity)-Discount) as decimal(10,2)) as cost from [order details] group by ProductID

7. Deleting Views DROP VIEW N.B If you have views created on tables, don't delete the tables without first deleting the views Note – create view must be the first line in a query. If it is not, use ‘GO’ before it.

8. Queries and Relational Algebra Databases

9. The Relational Algebra 5 main operations –Selection(Unary operation – on one table) –Projection(Unary operation – on one table) –Cartesian Product(Binary operation >1 table) –Union(Binary operation >1 table) –Set Difference(Binary operation >1 table) Plus derivable: –Join –Intersection

10. Query essentials Selection –This is where rows from a table are extracted on the basis of the value of one or more of the fields Projection –This is where columns from a table are extracted on the basis of the value of one or more of the fields Join –This is where rows and columns from more than one table are extracted

11. Selection –“The Selection operation works on a single relation R and defines a relation that contains only tuples of R that satisfy the specified condition (predicate)” Example: Print ' ' Print ‘***************************************' Print 'Selection on relation STOCK :predicate Stock_level > 30' Print '***************************************' Print ' ' Select * from stock where stock_level > 30

12. Projection –“The projection operation works on a single relation R and defines a relation that contains a vertical subset of R, extracting the values of specified attributes and eliminating duplicates.” Print 'Projection on relation STOCK taking only stock code, description and stock level' Print '*************************************************' Print ' ' Select Stock_code, Stock_Description, Stock_Level from Stock

13. Cartesian product –The Cartesian product of Order_Line x SupplierOrderLine is the table whose attribute set contains the fully qualified attribute names of all attributes from both tables. Set up a query with * from both tables. Select * from Order_Line, SupplierOrderLine

14. Equi-join –Rows are combined if there are equal values in certain selected columns from each table. Select * from Stock join Supplier on Stock.Supplier_Id = Supplier.Supplier_Id

15. Natural join This is where two tables may have several columns in common. ALL common columns are included in the join. The tables need not be related. Select * from Order_Line join SupplierOrderLine on SupplierOrderLine.Stock_Code = Order_Line.Stock_Code Note: Natural joins can be implemented using ‘where’ Select * from Order_Line, SupplierOrderLine where order_line.stock_code = SupplierOrderLine.stock_code

16. Intersection The intersection can be derived using a natural or equi-join. Select order_line.stock_code,order_line.quantity, supplierorderline.stockrequired from order_line join supplierorderline on order_line.stock_code = supplierorderline.stock_code

17. Theta join This is similar to an equi-join but the condition can be other than =. It can be:- –equal to –not equal to –less than –less than or equal to –greater than –greater than or equal to

18. Sample Theta Join Select customer_amount_owing, customer_name, payment_amount from jcustomer join payment on jcustomer.customer_amount_owing < payment.payment_amount

19. Union – S and T are tables with the same attributes –union S  T includes all rows from S and all rows from T –Print ' ' –Print '*****************************************' –Print 'Union of relations Order_Line and SUPPLIEROrderLine ' –Print '***Note that the SupplierOrderNo is coming out under the Quantity heading' –Print '*******************************************' –Print ' '; –Select * from Order_Line union Select * from SupplierOrderLine

20. Null value Null represents a value for an attribute that is currently unknown or is not applicable for this tuple. When checking for null, you should not use ‘=’ or ‘like’ When checking for null, use –Is null –Is not null

21. Semi joins This is where all of the rows from one table and any matching rows from the other table are shown. In a join the left table is the table on the left – i.e the one that is mentioned first, before the word join. The right table is the table that comes after the word join. An outer join can be a left join or a right join.

22. Semi joins contd. A left join returns all rows from the left table and matching rows from the right. A right join returns all rows from the right table and matching rows from the left. Where there are no matching rows, NULL values are returned. Select * from stock left join order_line on stock.stock_code = order_line.stock_code ;

23. Difference The difference between sets A and B is the rows that appear in A, but not B. The difference between STOCK and ORDER_LINE is the stock that has never been ordered. Difference can be returned one of two ways: –Using an outer join –Using a sub-query

24. Difference with outer join Find all stock items and the orderlines for them. Eliminate any rows that have a non-null stock code. Select stock.* from stock left join order_line on stock.stock_code = order_line.stock_code where order_line.stock_code is null;

25. Difference with sub-query The inner query returns stock codes that have been ordered. The outer query selects stock code that are not in the returned set of ordered stock codes. Select * from stock where stock_code not in (Select Stock_code from order_line)

26. Full Outer Join This is where the tables are joined, but if there is a row in either table that has no corresponding row in the other table, the attributes are shown as null. E.g. Match stock that was bought in from the supplier against stock that was sold: select order_line.stock_code as 'sold',order_line.quantity, supplierorderline.stock_code as 'bought', supplierorderline.stockrequired from order_line full outer join supplierorderline on order_line.stock_code = supplierorderline.stock_code