JOINS cis 407
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JOINS cis 407 Inner join Right and left outer join Full join Cross join
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JOINS cis 407. Inner join Right and left outer join Full join Cross join. Cross Product. P PS 101 nut 101 acme 102 bolt 101 wacky 102 acme - PowerPoint PPT Presentation
Transcript of JOINS cis 407
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
