MySQL date calculation in WHERE clause

Question

I want to get a statistic of registered users per month for the past year, so I constructed this query

SELECT MONTH(created_at) as m, COUNT(*) as c
FROM users
WHERE created_at >= DATE_FORMAT(DATE_SUB(NOW(), INTERVAL 12 MONTH), '%Y-%m')
GROUP BY m

Now what I understand MySQL will do is

• Select all rows
• Perform conditional check - if field created_at is gte to DATE_FORMAT(DATE_SUB(NOW(), INTERVAL 12 MONTH), '%Y-%m') add row to temporary table, else disregard it
• Group rows based on the derived column m
• Count rows in each group and put them in derived column c

If supposedly what I said above is correct, I am concerned about point 2. In a typical program the value in the if statement will be calculated every time if not put in a variable.

Do I need to put the statement DATE_FORMAT(DATE_SUB(NOW(), INTERVAL 12 MONTH), '%Y-%m') in a variable and reference that in my WHERE clause to keep it from recalculating it on each row, and how?

Answer 1

While executing your query the very first part that will be considered is the where clause. After that group by and then It will select all those resulting rows.

You don'y need to add any extra variables you just talked about at the end