Joins

1. Joins

2. Relational algebra • Recall relational algebra was the study of actions that are performed on one or more tables and give as a result another table. • The action is called an operation. • The things acted upon (tables in this case) are known as operands.

3. Basic Operations • The basic operations were • Selection: picking rows that satisfy some condition (predicate) from the table. • Projection: picking columns from the table. • Union, intersection and set difference: basic set operations that apply to union-compatible tables. • Cartesian product: concatenate two rows, one from each table; make all such combinations.

4. The Join Operation • An inner join of two tables is a Cartesian product operation followed by a selection operation (and possibly followed by a projection operation). • If one straightforwardly implements a join, the Cartesian product intermediary can be huge. • On the other hand, an earlier introduction of the selection condition may require a lot of searching (for matches). • This is a reason that relational database management systems (RDBMs) can exhibit performance problems.

5. Variations of the join operation • Theta join • Equijoin (a particular type of Theta join) • Natural join (a projection of an Equijoin) • Outer join (handles unmatched records differently) • Semijoin

6. Theta join (-join) • The restriction condition selecting from the Cartesian product does not have to be an equality, it could be any comparison operator such as • Greater than (>) • Greater than or equal to (>=) • Less than (<) • Less than or equal to (<=) • Not equal to (<>) • Using general condition to restrict the Cartesian product is known as a Theta join. • R FS (R and S are tables, F is a condition)

7. Theta Join Example • You have a table of customers who have a budget. • You have a table of items which have a price. • You want to advertise your items to customers who can afford them. • The desired relationship is an inequality, a person’s budget should be greater than the price of the item.

8. Theta Join Example: Advertising to Customers who can afford an item The tables Note that both have fields called ID, Access may be fooled into thinking this is the basis for a relationship.

9. Theta Join Example: Advertising to Customers who can afford an item Right click on relationship line to eliminate.

10. Theta Join Example: Advertising to Customers who can afford an item Choose fields to be displayed (projection).

11. Theta Join Example: Advertising to Customers who can afford an item No condition imposed yet, just a Cartesian product with projection.

12. Theta Join Example: Advertising to Customers who can afford an item Cartesian product projected but not restricted.

13. Theta Join Example: Advertising to Customers who can afford an item Condition added. Since it’s an inequality, this is a Theta Join. Also added Group By so the results would be grouped by Item.

14. Theta Join Example: Advertising to Customers who can afford an item

15. Theta Join Example: Advertising to Customers who can afford an item

16. Equijoin • The Equijoin is a special case of the Theta join in which the restriction condition is equality. • Example: a list of orders and the people placing them.

17. Equijoin Example: a list of orders and the people that placed them

18. Equijoin Example: a list of orders and the people who placed them Condition is equality, making this an Equijoin.

19. Equijoin Example: a list of orders and the people who placed them Order.CustomerID matches Customer.CustomerID even though Access is showing lastnames instead.

20. The Natural Join • Note that the previous join had both of the matching columns (Order.CustomerID and Customer.CustomerID) • A join that projects out one of the matching columns is known as a Natural Join.

21. Natural Join Example (using Wizard)

22. Natural Join Example (using Wizard)

23. Natural Join Example (using Wizard)

24. Natural Join Example (using Wizard) Projecting out matching column is what makes this a Natural join.

25. Natural Join Example (using Wizard) Does counts, totals etc. instead of listing individual records.

26. Natural Join Example (using Wizard)

27. Natural Join Example (using Wizard) Where’s Betty Rubble?

28. Semijoin • Not all of the Customers have matches in the Order Table. • By match we mean they have no order with that particular CustomerID. • If we select out those rows from the Customer table that do have a match in the Order table, we have a Semijoin. • Semijoins can be useful in distributed systems. You can cut down on the amount of information you send across the network. • There may be more processing at the other end.

29. Semijoin: Customer Orders Two tables joined, but only one displayed in results. A semijoin.

30. Semijoin: Customers who have placed orders Jane Doe appears twice.

31. Semijoin: Customers who have placed orders (SQL View)

32. Semijoin: DISTINCT customers who have placed orders (SQL View)

33. Semijoin: DISTINCT customers who have placed orders (DataSheet View)

34. Outer Join: Bringing Back Betty • All of the previous Equijoins have been what are called Inner Joins. • If a record from one table does not have a match in the other table, it is eliminated. • If this elimination feature is not desired, then you want to use an Outer Join. • The Outer Join keeps records that do not have matches. • R S

35. Access Help: Join Type

36. Inner Join: Customers and orders Inner Join

37. Inner Join: Customers and orders

38. Inner Join: Customers and orders Still Inner

39. Converting to Outer Join: Right Click on Relationship Line and choose Join Properties

40. Join Properties dialog box

41. Outer Join: Customers and orders Was a line, now is an arrow

42. Outer Join: Customers with or without orders

43. Outer Join: Customers and orders Customers who have not placed orders.

44. References • Database Systems, Rob and Coronel • Database Systems, Connolly and Begg