Joins CSC 240 Blum 1 Relational algebra Recall

Joins CSC 240 (Blum) 1

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. CSC 240 (Blum) 2

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. CSC 240 (Blum) 3

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. CSC 240 (Blum) 4

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 CSC 240 (Blum) 5

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) CSC 240 (Blum) 6

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. CSC 240 (Blum) 7

Theta Join Example: Advertising to Customers who can afford an item The tables Note that both have fields called ID, Access will be fooled into thinking this is the basis for a relationship. CSC 240 (Blum) 8

Theta Join Example: Advertising to Customers who can afford an item Right click on relationship line to eliminate. CSC 240 (Blum) 9

Theta Join Example: Advertising to Customers who can afford an item Choose fields to be displayed (projection). CSC 240 (Blum) 10

Theta Join Example: Advertising to Customers who can afford an item No condition imposed yet, just a Cartesian product with projection. CSC 240 (Blum) 11

Theta Join Example: Advertising to Customers who can afford an item Cartesian product projected but not restricted. CSC 240 (Blum) 12

Theta Join Example: Advertising to Customers who can afford an item Also added Group By so the results would be grouped by Item. CSC 240 (Blum) Condition added. Since it’s an inequality, this is a Theta Join. 13

Theta Join Example: Advertising to Customers who can afford an item CSC 240 (Blum) 14

Theta Join Example: Advertising to Customers who can afford an item CSC 240 (Blum) 15

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. CSC 240 (Blum) 16

Equijoin Example: a list of orders and the people that placed them CSC 240 (Blum) 17

Equijoin Example: a list of orders and the people who placed them Condition is equality, making this an Equijoin. CSC 240 (Blum) 18

Equijoin Example: a list of orders and the people who placed them CSC 240 (Blum) Order. Customer. ID matches Customer. ID even though Access is showing lastnames instead. 19

The Natural Join • Note that the previous join had both of the matching columns (Order. Customer. ID and Customer. ID) • A join that projects out one of the matching columns is known as a Natural Join. CSC 240 (Blum) 20

Natural Join Example (using Wizard) CSC 240 (Blum) 21

Natural Join Example (using Wizard) CSC 240 (Blum) 22

Natural Join Example (using Wizard) CSC 240 (Blum) 23

Natural Join Example (using Wizard) Projecting out matching column is what makes this a Natural join. CSC 240 (Blum) 24

Natural Join Example (using Wizard) Does counts, totals etc. instead of listing individual records. CSC 240 (Blum) 25

Natural Join Example (using Wizard) CSC 240 (Blum) 26

Natural Join Example (using Wizard) Where’s Betty Rubble? CSC 240 (Blum) 27

Semijoin • Not all of the Customers have matches in the Order Table. – By match we mean they have no order with that particular Customer. ID. • 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. CSC 240 (Blum) 28

Semijoin: Voiced Characters Two tables joined, but only one displayed in results. A semijoin. CSC 240 (Blum) 29

Semijoin: Customers who have placed orders Jane Doe appears twice. CSC 240 (Blum) 30

Semijoin: Customers who have placed orders (SQL View) CSC 240 (Blum) 31

Semijoin: DISTINCT customers who have placed orders (SQL View) CSC 240 (Blum) 32

Semijoin: DISTINCT customers who have placed orders (Data. Sheet View) CSC 240 (Blum) 33

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 CSC 240 (Blum) 34