JOINScis 407

Inner join

Right and left outer join

Full join

Cross join

Cross Product• P<Pnum, Pname> PS <Pnum, Supplier>

101 nut 101 acme 102 bolt 101 wacky 102 acme

• PxPS <Pnum, Pname, Pnum, Supplier> 101 nut 101 acme 101 nut 101 wacky

101 nut 102 acme 102 bolt 101 acme 102 bolt 101 wacky

102 bolt 102 acme• SQL: select companyName, shipCity

from customers, order• But only want result rows where part numbers match

Inner Joins (typically what is meant by join)

• Select *from customers c inner join [order details] od on c.customerID = od.customerID

• Old methodSelect * from customers c, [order details] odwhere c.customerID = od.customerID

Example #1

• Select s.companyName, p.productNamefrom products p join suppliers s on p.supplierid = s.supplieridwhere s.city = ‘city’ and p.discontinued = TRUE

• ‘ON’ specifies how relations joined

• ‘where’ constrains what is in result

Example #2• Select c.companyName, p.ProductName

from suppliers s join products p on s.supplierID = p.supplierID join [order details] odon p.productID = od.productID joinorders o on od.orderid = o.orderid joincustomers c on o.customerID = c.customerIDwhere c.city = s.city

• Generate an equivalent English sentence

OUTER JOIN• A left or right outerjoin is something like a

combination of a join and a crossproduct.

• (pg 90) a left outer join includes all the information from the table on the left.

• There many be a lot of nulls in the result.

Example• Select companyName, orderID

from company c inner join order o on c.customer = o.customer– Only customers that have ordered products

will be included in the result.

• Select companyName, orderIDfrom company c left outer join order o on c.customer = o.customer– Includes all customers will null orderIDs in

nothing ordered.– Outer joins useful in finding orphan records

Null does not match Null

• A value of null does not join to a value of null. Why?– x = null will not work– X is null will work

• Select c.customerid, companynamefrom customer c left outerjoin orders oon c.customerid = o.customeridwhere o.customerid is null

Other joins

• Full outerjoin combines left and right outerjoin

• Cross join is cross product.– May be useful in populating test database

Other examples

• Select * from customers c, orders owhere c.customerid = o.customerid

• Select * from customers c, orders owhere c.customerid != o.customerid

UNION• Select companyName

from customerswhere city = ‘NY’unionselect companyNamefrom Customerswhere city = ‘Pocky’

• Union selects must have the same number of columns. Must be the same domain for results to make much sense. (pg 105)

• See example pg 106