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Computer science · University

Relational data, SQL and safe queries

A relational table holds rows and columns with a defined schema. Keys identify rows or relationships, while constraints enforce some consistency rules.

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Prerequisites

Relevant school subject foundations and careful reasoning.

Study notes

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.

Worked examples

Which clause specifies result ordering in an ordinary SQL query?

Without explicit ordering, result order should not be assumed.

ORDER BY

A table contains 12 rows and a stated filter matches 5. How many rows does SELECT without grouping return for that filter?

The filter retains exactly the five matching rows.

5

Check your understanding

  1. What is a primary key intended to do?

    1. Uniquely identify rows under its constraint
    2. Make every column secret
    3. Sort every query automatically
    4. Guarantee that all text is accurate

  2. Why can joining a parent table to several matching child records repeat the parent values?

  3. Why use parameterized SQL for user-supplied values?

    1. Remove all need for permissions
    2. Make every query return one row
    3. Keep values separate from executable SQL syntax
    4. Automatically correct every schema

  4. Three parent rows each match two child rows under the stated join. How many joined rows result?

Answers and marking guidance

Answers and marking notes
  1. Uniquely identify rows under its constraint

    Uniqueness and non-null key constraints identify records, not all semantic truth.

  2. Each matching parent-child combination appears in the join result. A one-to-many relation naturally produces multiple rows.

    Credit join cardinality.

  3. Keep values separate from executable SQL syntax

    Parameter binding reduces injection risk from data values.

  4. 6

    3×2=6 combinations.

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