MySQL Where-clause with select - mysql

Question

I have a table that has three columns: visitations, country and user_id. I have a query to retrieve amount of all visitors per country and re-visitors per country. Now, I would like to alter my query so, that I get both amounts and a ratio of re-visitors per country (re-visitors / all visitors). I'm just learning MySQL and I feel that I don't know the tools to select all visitors and re-visitors and then use them in the ratio. Is there a way to do this in one query? Could someone help me with this? Thanks!
Here is my query for all visitors (and re-visitors if the # is removed)
SELECT sum(Visitations) AS "Amount", country
FROM E91
#WHERE Visitations > 1
GROUP BY Country
ORDER BY `Amount` DESC
Sample of the data, user is a revisitor if visitations is above 1:
user_id | country | visitations
---------|--------------|---------------
beth123 | Germany | 4
david78 | USA | 2
matt23 | UK | 1
...

Answer 1

The where clause starts to filter records, leaving out records you do want to count for your total. To count the revisitors you can use CASE WHEN... END:
SELECT
sum(Visitations) AS "Amount",
sum(CASE WHEN Visitations > 1 THEN 1 ELSE 0 END) as "Re-Visitors",
country
FROM E91
GROUP BY Country
ORDER BY `Amount` DESC
For further use you could do something like this:
SELECT
sum(Visitations) AS "Amount",
sum(CASE WHEN Visitations > 1 THEN 1 ELSE 0 END) as "Re-Visitors",
sum(CASE WHEN Visitations > 1 THEN 1 ELSE 0 END) / sum(Visitations) as "X",
country
FROM E91
GROUP BY Country
ORDER BY `Amount` DESC
or:
SELECT Amount, Re-visitors, "Re-visitors"/Amount, country
FROM (
SELECT
sum(Visitations) AS "Amount",
sum(CASE WHEN Visitations > 1 THEN 1 ELSE 0 END) as "Re-Visitors",
country
FROM E91
GROUP BY Country
ORDER BY `Amount` DESC
) x

Answer 2

The amount of visitors and re-visitors has nothing to do with the sum of the column visitations.
You can get the number of visitors (all user_ids who visited a country) by counting the number of user_ids for each country and the number of re-visitors by counting the number of user_ids for each country when the column visitations is greater than 1:
SELECT country,
SUM(visitations > 1) AS revisitors,
COUNT(*) AS visitors,
AVG(visitations > 1) AS ratio
FROM E91
GROUP BY Country