Inner Join vs. Outer Join Information Retrieval from Relational Databases

Query Languages Relational Algebra Structured Query Language (SQL) Three main operators: Select, Project, Join Provides the conceptual basis for SQL and QBE Structured Query Language (SQL) The user enters commands according to a pre-defined syntax to retrieve desired data. Query By Example (QBE) The user starts with a sample of the table(s) columns and marks the fields he or she wants to include in the answer. Defaults are available for summarizing and manipulating the data.

SQL (Structured Query Language) Each query statement follows the same structure: SELECT attribute name(s) FROM table name(s) WHERE criteria is met;

Relational Algebra Select Project Join includes only certain rows from a database table in its “answer”. Project includes only certain columns from a database table in its “answer” Join combines two or more database tables on the basis of one or more common attributes

Join Types Inner join(default in access) Outer join includes only the records from both tables that have the exact same values in the fields that are joined I.e., Outer join includes all records from one table, and matches those records from the other table for which values in the joined fields are equal Left Outer Join Right Outer Join

Example Tables

Relationship View

Query to List ALL Employees and Description of any Training Courses Taken ( Inner Join)

Note: We have 6 employees Example Tables Note: We have 6 employees

Does this This Give Us What we Need? The above based upon the default join which is the inner join and will not give us employees that have not taken training classes SQL code created by access for QBE on previous slide

Using Outer Joins Note change

SQL code created by access for QBE on previous slide

The Revenue Cycle of M&M M&M sells its agleclaps to customers through a network of company salespeople. Each type of agleclap is bought from a particular vendor and is given an initial list price. Each salesperson services a separate group of customers and is allowed to offer them various discounts from list to induce sales. Each sale can include one or more types of agleclaps and can be paid for in any one of three ways: (1) immediately in cash, (2) on the 15th of the following month, or (3) over the course of six months. When cash is received, a cashier deposits it into a company bank account. Sales are signaled by invoices; cash receipts by remittance advices

Relationship View

Example Tables (Incomplete Enterprise Database) from Dunn & McCarthy (2004) working paper

SQL and Relational Algebra Inner Join vs. Outer Join Find all details of all sales and the cash receipt number and amount applied of any cash receipts related to those sales Data we need for this example is in 2 tables Summary sales totals have been stored in sales table If summary figure not stored, you would need also need the sale-inventory relationship table to calculate sales totals Note: there have been no cash receipts related to sales transaction 6 & 7. We examine here the same example for which we previously needed a Left Outer Join of Sale and Sale-CashRecDuality. The equivalent SQL statement and query result are shown. Notice that the Left Outer Join is accomplished in the FROM and WHERE clauses of the SQL statement.

Relational Algebra Inner Join in QBE Details of all sales, related cash receipts

Relational Algebra Inner Join in QBE Details of all sales, related cash receipts

Default join is inner Join If you double click on the join you will see the join properties box. Item 1, the inner join is the default

Inner Join Results Sales transactions 6 & 7 are not listed since there was no cash receipts associated with them. Does this create a potential problem?? SELECT Sale.SaleNumber, Sale.Amount, Sale.Date, Sale.CustomerNumber, [Sale-CashRecDuality].RemittanceAdviceNumber, [Sale-CashRecDuality].Applied FROM Sale INNER JOIN [Sale-CashRecDuality] ON Sale.SaleNumber = [Sale-CashRecDuality].SaleNumber;

Relational Algebra Outer Join in QBE Details of all sales, related cash receipts Double-click on the join line

Relational Algebra Outer Join in QBE Details of all sales, related cash receipts Click on appropriate join type Click OK

Relational Algebra Outer Join in QBE Details of all sales, related cash receipts Note change in Join line

Relational Algebra Outer Join in QBE Details of all sales, related cash receipts Result

Joins Compared Outer Join Inner Join