Question

. Consider the following relations for a database that keeps track of business trips of salespersons in a sales office: SALESPERSON(SSN, Name, Start Year, DepcNo) TRIP(SSN, From_City, To_City, Departure_Date, Return_Date, Trip ID) EXPENsE(Trip ID, Accountg, Amount) Specify the foreign keys for this schema, stating any assumptions you make.

Short answer

The foreign keys in this database schema are SSN in the TRIP table and Trip ID in the EXPENSE table.

Step-by-step solution

Identify Primary Keys

The first task is to identify the primary keys of each table. The primary keys are unique identifiers for each record in a table. For the SALESPERSON table, SSN (Social Security Number) serves as the primary key because it is unique for every salesperson. For the TRIP table, Trip ID appears to be the unique identifier, so it is the primary key. And for the EXPENSE table, the combination of Trip ID and Accountg could serve as the primary key, assuming each expense account is only used once per trip.

Foreign Keys in TRIP Table

A foreign key in a table is a field (or collection of fields) that is primary key in another table. In our case, in the TRIP table, SSN is a foreign key. It is the primary key in the SALESPERSON table and links a trip to a specific salesperson.

Foreign Keys in EXPENSE Table

In the EXPENSE table, Trip ID is a foreign key because it is the primary key in the TRIP table. This foreign key relationship implies that each expense is associated with a specific trip.

Key concepts

Primary Keys

When designing a database schema, one of the essential elements is the primary key. A primary key is an attribute or a set of attributes that uniquely identify a record within a database table. This means no two rows in a table can have the same primary key value. Choosing a good primary key is crucial because it ensures the integrity of the data and helps in effectively managing relationships between tables.

In our exercise, we have different tables, each requiring its primary key:

• SALESPERSON Table: The Social Security Number (SSN) is the primary key. Each salesperson has a unique SSN, making it a perfect candidate.
• TRIP Table: The Trip ID is the primary key. This ensures that each trip record is unique.
• EXPENSE Table: The combination of Trip ID and Accountg serves as the primary key, assuming each accounting entry is unique per trip.

By correctly identifying and using primary keys, you establish a firm backbone for your data structure which is fundamental for robust database management.

Foreign Keys

Foreign keys are another critical concept in relational databases. While primary keys ensure uniqueness within a table, foreign keys create relationships between tables. A foreign key in one table points to a primary key in another, effectively linking the two tables.

In the context of the given exercise, foreign keys help maintain referential integrity:

• TRIP Table: The SSN is a foreign key that links to the SALESPERSON table. This relationship indicates that each trip is associated with a specific salesperson.
• EXPENSE Table: The Trip ID serves as a foreign key linking to the TRIP table, signifying that each expense is connected with a particular trip.

Using foreign keys ensures that all data references across tables are accurate and valid, preventing orphan records and ensuring that relationships reflect the real-world actions they model.

Relational Database

A relational database organizes data into tables, which can be linked—or related—based on data common to each, like a primary and foreign key. This structure is designed to ensure data consistency and integrity while allowing complex queries to extract meaningful insights.

The tables in a relational database are not standalone entities; they are interconnected. In this exercise, you have three primary entities represented by tables: SALESPERSON, TRIP, and EXPENSE. Each table's design is integral to the overall database schema, and how they relate to each other sets up how data is accessed and modified.

• SALESPERSON Table: Contains individual records about salespersons.
• TRIP Table: Keeps records of different trips each salesperson takes.
• EXPENSE Table: Documents expenses incurred during each trip.

The relational database model, by organizing data this way, makes it easier to facilitate precise data retrieval and efficient data management. This structure enables users to define relationships in such a way that they can easily retrieve, insert, update, and delete information in a streamlined and secure manner.