Hello, this is my first technical english post so do not read it very criticaly. I want to describe interesting task with SQL which I faced with.

I have a table with next columns: id, email, etc.

I need to calculate quantity of each e-mail domain, calculate percent of domain and sort this information.

Solution:

SELECT
    domain,
    total,
    (total / emailscount) * 100 AS domainpercentage
FROM
    (
        SELECT
        SUBSTRING_INDEX(email, '@', -1) AS domain,
        COUNT(contacts_list.id) AS total,
        (
            SELECT COUNT(cl.id)
            FROM contacts_list AS cl
        ) AS emailscount
        FROM contacts_list
        GROUP BY domain
        ORDER BY total DESC
    ) AS result

Explanation:
At first I select all domains and quantity of their entries.

SELECT
    SUBSTRING_INDEX(email, '@', -1) AS domain,
    COUNT(contacts_list.id) AS total,
    (
        SELECT COUNT(cl.id)
        FROM contacts_list AS cl
    ) AS emailscount
FROM contacts_list
GROUP BY domain
ORDER BY total DESC

Also I select amount of all entries in my table via inner query.

(
    SELECT COUNT(cl.id)
    FROM contacts_list AS cl
) AS emailscount

Then I wrap my query where I select inner`s query results and calculate percentage of each domain.

SELECT
    domain,
    total,
    (total / emailscount) * 100 AS domainpercentage
FROM
(...
) AS result

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