Lære Subqueries for Aggregation and Filtering | Subqueries in Library Management

Subqueries are powerful tools in SQL that allow you to perform advanced operations, such as aggregation and filtering, within your queries. In a library management system, you often need to answer questions like "Which members have borrowed more than a certain number of books?" or "Which books are the most popular?" Subqueries let you break down these problems by first calculating an aggregate value and then using that result to filter or further process your data.

To use subqueries for aggregation, you can nest a SELECT statement that performs an aggregate function—such as COUNT, MAX, or MIN—inside another query. This inner query computes a summary value, which the outer query can then use to filter results or add context. For example, you might count the number of borrowings per member and then select only those members who meet a certain threshold.

Filtering with subqueries is similar. You can use the result of a subquery in the WHERE clause to include or exclude rows based on complex conditions. This is especially useful when you want to compare each row to an aggregated value or a set of values produced by a subquery.


              12345678
            
-- Find members who have borrowed more than 5 books
SELECT m.member_id, m.name
FROM members m
WHERE (
    SELECT COUNT(*)
    FROM borrowings b
    WHERE b.member_id = m.member_id
) > 1;

1. Which SQL clause is most commonly used with a subquery to perform aggregation, such as counting borrowings per member?

2. Which of the following queries correctly filters members who have never borrowed a book using a subquery?

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Suggested prompts:

Can you explain how this subquery works step by step?

How can I modify this query to find members who borrowed more than 10 books?

What are some common mistakes to avoid when using subqueries in SQL?

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              12345678
            
-- Find members who have borrowed more than 5 books
SELECT m.member_id, m.name
FROM members m
WHERE (
    SELECT COUNT(*)
    FROM borrowings b
    WHERE b.member_id = m.member_id
) > 1;

1. Which SQL clause is most commonly used with a subquery to perform aggregation, such as counting borrowings per member?

2. Which of the following queries correctly filters members who have never borrowed a book using a subquery?

Var alt klart?

Tak for dine kommentarer!

Sektion 3. Kapitel 1