Relational Databases and SQL
What's covered
Key facts
The best way to avoid storing a customer's address twice is to store the address once in a Customers table and link to it from Orders via a foreign key.
A database is an organised collection of data.
A flat-file database stores all data in a single table (or file) without explicit links to other tables.
A composite primary key uses two or more columns together for uniqueness — useful when no single column is unique on its own.
In AQA-style GCSE SQL, two tables are joined using a WHERE clause that matches the key fields: ... FROM A, B WHERE A.id = B.a_id.
DELETE FROM does NOT remove the table itself from the database.
INSERT INTO Books DEFAULT VALUES inserts a row using each column's declared default value (where defined).
ASC stands for ascending order — smallest to largest, or A→Z for text.
The FROM keyword specifies the table (or tables) the query retrieves data from.
The SET clause specifies the new field values to assign.
Sample questions
A taste of the 121 questions in this topic — answers marked. Sign up to practise the full set with spaced repetition.
What is data redundancy?
- •Data stored across multiple tables using foreign keys
- •Data that has been encrypted and backed up
- •Missing values in a database record
- ✓Same data stored more than once
Why use a database instead of separate files?
- •Always smaller in size than text files
- ✓Better organised and easier to query
- •Removes the need for storage devices
- •Runs much faster than the CPU only
How are tables in a relational database linked?
- •Through random row order in each table
- ✓Through shared keys between tables
- •Through the file size of each table
- •Through the names of the columns alone
What does a primary key guarantee?
- •Each field contains unique values too
- •Each query returns the same data always
- ✓Each record is uniquely identifiable
- •Each table is linked to every other one
What links two tables in a relational query?
- •A common SQL alias
- ✓A matching foreign key
- •A shared table name
- •An identical column data type
Which SQL removes Sam from Students?
- ✓DELETE FROM Students WHERE name = 'Sam'
- •DELETE Sam FROM Students table now
- •DROP Students WHERE name = 'Sam' only
- •REMOVE FROM Students WHERE name = 'Sam'
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