Lukas' Notes

SELECT (SQL)

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sql

Definition

SELECT (SQL)

A SELECT statement in SQL is used to retrieve data from one or more database tables, or expressions. It allows you to specify which columns you want to see, from which tables, and can include conditions to filter the rows returned. The result of a SELECT statement is a temporary table called a result set.

Relational Division Problem

The relational division problem in SQL involves finding entities in one table that are related to all entities in another table, typically through an intermediary linking table. For example, finding “students who have taken all mandatory courses” or “suppliers who supply all parts required for a specific product.” This type of query is often solved using techniques like double NOT EXISTS clauses or by ensuring the count of matched related entities equals the total count of entities in the target set.

Example: Find students who are enrolled in all courses designated as ‘Computer Science’.

This goal can be translated to: ‘Find students for whom there are no ‘Computer Science’ courses in which they are not enrolled’. This “natural language query” can be easier translated into a SELECT statement:

SELECT s.name
FROM students s
WHERE NOT EXISTS (
	SELECT 1
    FROM courses c
    WHERE c.category = 'Computer Science'
      AND NOT EXISTS ( 
        SELECT 1
        FROM enrollments e
        WHERE e.studentId = s.studentId
          AND e.courseId = c.courseId
    )
);

According to De Morgan’s rules:

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