Quick questions on Relational databases and SQL queries in HSC Information Processes and Technology

A relational database organises data into two-dimensional tables, also called relations. Each table describes one kind of thing, for example Students or Subjects. A row in the table is a record, representing one instance (one student). A column is a field, representing one attribute (the student's surname).

Normalisation is the process of organising fields into tables so that each piece of data is stored once. Without it, repeating a customer's address on every order wastes space and risks inconsistency when the address changes in one place but not another. Normalisation splits data into related tables linked by keys, removing this redundancy and the update anomalies it causes.

SQL (Structured Query Language) is the standard language for querying relational databases. A basic retrieval has three clauses:

When the data you need spans two tables, a join combines them on a matching key. To list each student with the names of the subjects they take, you join Student to Subject through the Enrolments linking table, matching StudentID to StudentID and SubjectID to SubjectID. The join is the SQL expression of a relationship.

Beyond retrieval, SQL updates the stored data. INSERT adds a new record, UPDATE changes fields in existing records (usually with a WHERE clause so it does not change every row), and DELETE removes records. These map onto the processing and storing information processes.