SQL212.2 Introduction to SQL using Oracle Module 2

Bookstore2 SQL212 Module 2 1

SQL212

SQL Programming

Workshop 2 – Joins, Subqueries, Unions, Calculations and Grouping


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SQL212

SQL Programming

Part 1 – Joins, Subqueries


Relational Database with constraints (from text)


Warning!

• Some slides may show queries using a table called…

Order_filled

• The current database uses a better name…

Orders

Construct your queries with this latter table name


Joins

• Inner• Outer

– Left– Right– Full

• Cross• Self• Theta• We will cover the most important; others as time

and interest permit


Inner Join

• Pairs each row from first table with corresponding row from second table over the “join column”

• The result set only contains rows where there is a match over the join column in both tables

• Equi-join is the common inner join


Inner Join

Older Syntax:

Select <column-list>

From <tablelist>

Where <predicate>

Still very commonly used

Inner Join Examples

• Show customers and their orders

• Join the customers table to the orders table where the customer number in each table matches



Inner Join

Example using older syntax:

SELECT customer_first_name, customer_street, order_numb, order_date

from customers, orders

Where customers.customer_numb = orders.customer_numb


Inner Join with Result


Inner Join (New Syntax)

Basic SQL 92 Syntax:


From <table1>

Inner join <table2>

On <join condition>


Inner Join

Basic Example:


from customers

inner join orders

on customers.customer_numb = orders.customer_numb


Inner Join with Result


Inner Join over Multiple columns

• Note that that the join condition can apply to multiple columns if desired

• Used with composite keys


Inner Join Result in MS Access


Inner Join

• In the last example…– What was the cardinality of the relationship

between customers and orders?– Which table was the parent?– What was it’s primary key?– In which table did we employ a foreign key

and what was it?


Cross Join

• What happens when you omit a join expression?

• Get the cartesian product of the tables – all possible combinations of the two tables

• For large tables this will run a long time!

Dual

• Sometimes we want to generate an expression without actually referring to a real table.

• A trivial example would show the date.

• Oracle has a 1 row, 1 column built in table called Dual to handle this.

• Ex: select sysdate from dual;



Cross Join Result Set in MS Access


Additional SQL92 Syntax

• Table1 natural join table2 – automatically uses columns with same name

• Table1 natural join table2 using(<column-list>)

• Not yet widely available in commercial implementations

Natural Join



Joining More than Two Tables

• Can join several tables in one select• Try to limit to three or four• Join order can be important for

performance (although optimizers will usually handle this for you)

• Use parentheses to force order of evaluation (also vendor extensions, often called “hints”)


Joining More than Two Tables

• Add orderlines detail to previous queries

SELECT customer_first_name, customer_street, orders.order_numb, orders.order_date, orderlines.isbn, orderlines.quantity

FROM customers

INNER JOIN orders ON

customers.customer_numb=orders.customer_numb

INNER JOIN orderlines

on orders.order_numb = orderlines.order_numb


Multi-table Join with Results


MS Access Multi-table Join Result Set

Own Your Own

• Add the book title to the previous query



Sample Database

• Before we continue (Access classes only)…

• Create a new employees table


Correlation Names (Table Aliases)

• Can abbreviate references to tables

• For example:Select e.name, j.payrange

From employees as e

Inner join job_information as j

On e.jobcode = j.jobcode;


Self Joins

• Implements a recursive relationship

• Important in various applications– Parts lists/assemblies– HR– Etc.– Table joined to itself using correlation names


Self Joins

SELECT e.*, m.name

FROM employees AS e, employees AS m

WHERE e.managerid = m.employeeid;



Outer Joins

• Left – selects all rows from the left or first table, even if no match exists in the other table– Widely used in commercial practice

– Especially useful for reporting

– Can be slower and interfere with optimizer

• Right – same idea but all rows from right table• Full – all rows form both tables


Left Outer Join

Basic SQL 92 Syntax:


From <table1>

Left [outer] join <table2>

On <join condition>

Outer Join Example

• Show all customers and their orders.

• Include customers with no orders as well.



Left-Join

Basic Example:


from customers as c

left join orders as o

on c.customer_numb = o.customer_numb



Left Join with Results


SQL200

SQL Programming

Part 2– Subqueries, Unions


Subqueries

• One select statement embedded in another.• Can be paced in select list, from clause or

where clause.– We will study the latter in this class

• Can be nested multiple levels deep• Two types:

– Uncorrelated – executes inner query then outer– Correlated – executes inner query once for each outer

query row

Subquery Example

• Show all the orderline information for orders places in 1999

• Note that orderlines table does not have order date. Thus must filter on a different table (orders).



Uncorrelated Subquery

select isbn, quantity

from orderlines ol

where ol.order_numb in

(select o.order_numb from orders o where order_date between ‘1-JAN-99’ and ’31-DEC-99’)


Uncorrelated Subquery with Results


Negative Subquery

• A type of subquery that matches “not found” conditions


Negative Subquery

select isbn, quantity

from orderlines ol

where ol.order_numb not in

(select o.order_numb from orders o where order_date between ‘1-JAN-99’ and 31-DEC-99’)


Negative Subquery with Results


Correlated Subquery with Exists

• Inner subquery executed once for each outer row• Exists will return true or false depending on

whether the result will have any rows or not• Can be a quick way to test for existence of

records (parent records, say) as used in application enforcement of referential integrity


Correlated subquery with Exists

SELECT isbn, quantity

FROM orderlines AS ol

WHERE exists

(select * from orders o where ol.order_numb = o.order_numb

and o.order_date between ‘1-JAN-99’ and ‘31-DEC-99’);

This type of query covered in intermediate SQL class


Unions

• Combines two tables

• Tables must be union compatible

• Uses:– Combine current tables with history– Combine information from tables with no

common join column


Unions

Select <column-list> from <table1>

Union [ALL]

Select <same-columns> from <table2>

Union Example

• Create a mailing list of customers and sources

• Use a union of the two tables, making sure the columns match



Unions

Union Results



SQL200

SQL Programming

Part 3 – Calculations, Aggregates


Calculated Fields

• Can add a column calculated from others

SELECT order_numb, quantity, cost_each, quantity*cost_each as extension

FROM orderlines


Calculated field in the Result



String Manipulation

• Concatenation

• Trim

• Substring

• Upper, Lower

• Etc. (various vendor extensions)


Concatenation

• Used for concatenated keys

• Useful to format reports

Basic syntax:

(Oracle, std) Field1 || Field2


Concatenation

select customer_first_name

|| ‘ ‘ || trim(customer_last_name)

as Name

from customers



Date Functions

• Numerous date functions• Often vendor specific• Often used:

– Take the year of a date– Take the month of a date– Add a month (not 30 days)– Etc.


Aggregate Functions

• Count

• Sum

• Min

• Max

• Avg

• Often used in conjunction with grouping


Aggregate Functions

Basic syntax:

Select <function>(<column>)From <table>Group by <column-list>Having <predicate>

Group by all columns to left of one(s) you want to aggregate


Aggregate Functions

SELECT orderlines.order_numb, Count(*) AS “Number of Order Lines” , Sum(orderlines.quantity) AS SumOfquantity, Sum(quantity * cost_each) AS extension

FROM orderlines

GROUP BY orderlines.order_numb

having count(*) > 1



Having vs. Where

• Having and Where clauses are similar but not the same

• Having removes groups after they are formed

• Where removes rows before groups are formed


Exercise

• List all customers and their orders– Name nicely formatted– With orders in the year of 1999– Show total order quantities and amounts– Only include orders with more than three

order lines


Exercise Result

[end module]

Notes


Notes


Notes


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SQL212.2 Introduction to SQL using Oracle Module 2

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