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JOINS cis 407

JOINS cis 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

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JOINS cis 407

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  1. JOINScis 407 Inner join Right and left outer join Full join Cross join

  2. 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 wacky102 bolt 102 acme • SQL: select companyName, shipCity from customers, order • But only want result rows where part numbers match

  3. 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

  4. 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

  5. Example #2 • Select c.companyName, p.ProductNamefrom 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

  6. 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.

  7. Example • Select companyName, orderIDfrom 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

  8. 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

  9. Other joins • Full outerjoin combines left and right outerjoin • Cross join is cross product. • May be useful in populating test database

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

  11. UNION • Select companyNamefrom 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

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