MySQL calculate percentages

Information is only useful, if you view it in a context. In data analysis you should avoid to only look at absolute numbers. For example you could just get the number of all users that have signed up in the previous month:

SELECT COUNT(*) as has_orderd
FROM users
WHERE users.has_ordered = true
AND YEAR(date_created) = YEAR(CURRENT_DATE - INTERVAL 1 MONTH)
AND MONTH(date_created) = MONTH(CURRENT_DATE - INTERVAL 1 MONTH)

Result:

has_ordered
450

But this number only gives you a certain amount of useful information. So you should look at that data, compared to all users that signed up the previous month:

SELECT COUNT(*) as total
FROM users
WHERE YEAR(date_created) = YEAR(CURRENT_DATE - INTERVAL 1 MONTH)
AND MONTH(date_created) = MONTH(CURRENT_DATE - INTERVAL 1 MONTH)

Result:

total
1100

If you want both data points next to each other, you can do the following:

SELECT 
    SUM(users.has_ordered = true) as user_with_orders, 
    COUNT(*) as total
FROM users
WHERE YEAR(date_created) = YEAR(CURRENT_DATE - INTERVAL 1 MONTH)
AND MONTH(date_created) = MONTH(CURRENT_DATE - INTERVAL 1 MONTH)

Result:

user_with_orders    total
450                 1100

And to calculate the percentage of all users that have signed up the last month, you can just do:

SELECT 
    SUM(users.has_ordered = true) as user_with_orders, 
    COUNT(*) as total,
    ((SUM(users.has_ordered = true)/COUNT(*)) * 100) as percentage
FROM users
WHERE YEAR(date_created) = YEAR(CURRENT_DATE - INTERVAL 1 MONTH)
AND MONTH(date_created) = MONTH(CURRENT_DATE - INTERVAL 1 MONTH)

Result:

user_with_orders    total   percentage
450                 1100    40.9