Computer information system cheat sheet

System components

An information system combines people, processes, data and technology. A software application is only one component.

A database does not by itself define how an organisation uses its data.

Relational keys

A primary key identifies a row. A foreign key expresses a relationship with a key in another table.

An order can store the identifier of its customer.

Data quality

Accuracy, completeness, consistency and timeliness are different dimensions of useful data.

A complete record can still contain an incorrect value.

Process modelling

A process model shows activities, decisions and flows. Separate the current process from a proposed improvement.

Identify a handoff before deciding which step to automate.

Entity relationships

A data model identifies entities, attributes and relationships. Cardinality describes how many records may be related; optionality describes whether a relationship is required.

One customer can place many orders, while each order may refer to one customer.

Normalisation

Normalisation organises relational data to reduce certain update anomalies. It uses functional dependencies and keys; splitting every field into a separate table is not the goal.

Store a customer’s contact details in a customer record rather than repeating them in every order.

Transactions

A transaction groups related operations into one logical unit. Atomicity, consistency, isolation and durability describe different properties of transaction processing.

A transfer should not permanently debit one account while silently omitting the corresponding credit.

Access and governance

A system needs rules for data ownership, permitted use, retention and access. Technical controls should support those rules and provide a record of important actions.

Assign editing rights according to a role rather than sharing one unrestricted account.

Worked example: a join

A customer table has one record for customer 7. An order table has three orders for customer 7. Joining on customer ID returns three matching order rows, not one deduplicated customer summary.

The relationship’s cardinality helps predict the number of rows in a query result.

Common mistake: a database is the whole system

An information system includes people, processes, data and technology. A technically correct database can still fail if procedures and responsibilities are unclear.

Evaluate the complete workflow from data entry through use, correction and recovery.

Check your understanding

Why can a join produce more rows than the customer table contains?

One customer can match multiple records in a related table, such as several orders.

Curriculum connections

Sources