Computer science · University
Relational data, SQL and safe queries · revision notes
A relational table holds rows and columns with a defined schema. Keys identify rows or relationships, while constraints enforce some consistency rules.
Key ideas and conditions
Relations
A relational table holds rows and columns with a defined schema. Keys identify rows or relationships, while constraints enforce some consistency rules.
Queries
SELECT retrieves values; WHERE filters rows. A result has no guaranteed order without an appropriate ORDER BY clause.
Joins
A join combines related records according to a condition. One-to-many relationships can create multiple result rows, so count assumptions must be checked.
Safety
Parameterized queries separate data from SQL syntax. Permissions, validation and transactions remain necessary for a complete application.
Explain without looking
- Why is moving this worksheet app from JSON files to SQL a code change rather than only a filename change?
- What groups related updates into an atomic unit when properly supported?
Further reading and review references
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