Views, Algebra

Temporary Tables

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 <view_name> [ (<column_list>) ] AS

SELECT <statement> [ (WITH CHECK OPTION); ]

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).

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.

Non-base fields

• Derived fields• Sum• Average• Count• Summing group fields

View from Northwind

Create view ProductIncome asSelect ProductID,

cast(sum((unitPrice*Quantity)-Discount) as decimal(10,2))

as cost from [order details] group by ProductID

Deleting Views

• DROP VIEW <viewname>

• 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.

Queries and Relational Algebra

Databases

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

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

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

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

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

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

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

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 onorder_line.stock_code = supplierorderline.stock_code

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

Sample Theta Join

Select customer_amount_owing, customer_name, payment_amount from jcustomer

join payment on jcustomer.customer_amount_owing <

payment.payment_amount

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

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

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.

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;

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

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;

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)

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 onorder_line.stock_code = supplierorderline.stock_code