SQL: Multiple Tables CIT 381

Sample Schema

General Form

Naming Attributes

Renaming

FROM and Cross Product SELECT customers.company, customers.cust_num, orders.cust, orders.order_num FROM customers, orders; FROM creates the cross product of two (or more) tables. Here, every row of customers is paired with every row of orders. Note: this particular example is from the order processing database used in the SQL text. Since customers has 21 rows and orders has 30, the result of the above query returns 21*30 = 630 rows.

COMPANYCUST_NUMCUSTORDER_NUM AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates AAA Associates Ace International Ace International

Joins What we want to do is compute joins, matching the shared attribute in the two tables. stud_ssn in student stud_ssn in enroll This is by far the most important and common operation.

Computation of Joins FROM student s, enroll e drives all pairs of rows from the two tables into the condition. WHERE s.stud_ssn=e.stud_ssn picks out those pairs that match on the desired attribute. This is sometimes called a natural join.

Long Join Chains Find the name of all classes taken by Alice. (We must look in three tables.) SELECT c.class_dept, c.class_num, c.class_title FROM student s, enroll e, class c WHERE s.stud_name=‘Alice’ AND s.stud_ssn=e.stud_ssn AND e.class_crn=c.class_crn;

Joins in Access SELECT CUSTOMERS.COMPANY, CUSTOMERS.CUST_NUM, ORDERS.ORDER_NUM FROM CUSTOMERS INNER JOIN ORDERS ON CUSTOMERS.CUST_NUM = ORDERS.CUST; MS Access, while allowing the queries on previous slides, also allows for a non-standard syntax (Jet SQL). An inner join (what we’ve seen), is to be distinguished from an outer join, which we will see soon.