= Number of product changes each employee has made in the last month
{{{#!sql
CREATE OR REPLACE FUNCTION get_employee_product_changes_last_month()
RETURNS TABLE (
    id BIGINT,
    employee_name TEXT,
    number_of_product_changes BIGINT
)
LANGUAGE plpgsql
AS $$
BEGIN
    RETURN QUERY
    SELECT
        e.id,
        p.name AS employee_name,
        COUNT(DISTINCT mc.date_and_time) AS number_of_product_changes
    FROM employees e
    JOIN personal p
        ON e.id = p.id
    LEFT JOIN makes_change mc
        ON e.id = mc.id
    LEFT JOIN change ch
        ON mc.date_and_time = ch.date_and_time
    WHERE
        mc.date_and_time >= date_trunc('month', CURRENT_DATE) - INTERVAL '1 month'
        AND mc.date_and_time < date_trunc('month', CURRENT_DATE)
    GROUP BY
        e.id,
        p.name
    ORDER BY
        number_of_product_changes DESC;
END;
$$;

}}}

== Relational Algebra
- E(id, date_of_hire)
- P(id, name, first_name, last_name, email, password, permissions, type, authorisation)
- MC(id, date_and_time, permission, type, authorisation)
- CH(date_and_time, product_code, changes_made)

**JOIN employees with their personal information:**
- J1 ← E ⨝E.id= P.id P

**JOIN employees with the changes they have made:**
- J2 ← J1 ⨝E.id = MC.id MC

**JOIN the changes with the corresponding product:**
- J3 ← J2 ⨝MC.date_and_time = CH.date_and_time CH

**SELECT only changes made during the last calendar month:**
- F1 ← σ
date_and_time ≥ START_OF_CURRENT_MONTH - 1 MONTH
∧
date_and_time < START_OF_CURRENT_MONTH
(J3)

**Calculate total number of distinct changes made by each employee:**
- R ← γid, name;
     COUNT(DISTINCT date_and_time) → number_of_product_changes
     (F1)

**Sort by total number of changes made:**
- R_final ← τnumber_of_product_changes DESC(R)