Using a correlated query:

  SELECT t.id,
         t.count,
         (SELECT SUM(x.count)
            FROM TABLE x
           WHERE x.id <= t.id) AS cumulative_sum
    FROM TABLE t
ORDER BY t.id

Using MySQL variables:

  SELECT t.id,
         t.count,
         @running_total := @running_total + t.count AS cumulative_sum
    FROM TABLE t
    JOIN (SELECT @running_total := 0) r
ORDER BY t.id

Note:

• The JOIN (SELECT @running_total := 0) r is a cross join, and allows for variable declaration without requiring a separate SET command.
• The table alias, r, is required by MySQL for any subquery/derived table/inline view

Caveats:

• MySQL specific; not portable to other databases
• The ORDER BY is important; it ensures the order matches the OP and can have larger implications for more complicated variable usage (IE: psuedo ROW_NUMBER/RANK functionality, which MySQL lacks)

You can use a temporary variable to calculate the cumulative sum:

SELECT  a.num,
   (@s := @s + a.num) AS cumulative
FROM ID a, (SELECT @s := 0) dm
ORDER BY a.num;

I believe you can obtain a rolling sum of the unique clicks without needing to resort to dynamic SQL:

SELECT
    t1.CREATED_AT,
    (SELECT SUM(t2.uniqueClicks) FROM
     (
         SELECT CREATED_AT, COUNT(DISTINCT IP, URL) uniqueClicks
         FROM CODE_EVENTS
         GROUP BY CREATED_AT
     ) t2
     WHERE t2.CREATED_AT <= t1.CREATED_AT) uniqueClicksRolling
FROM
(
    SELECT DISTINCT CREATED_AT
    FROM CODE_EVENTS
) t1
ORDER BY t1.CREATED_AT;

The subquery aliased as t2 computes the number of unique clicks on each given day which appears in your table. The distinct count of IP and URL is what determines the number of clicks. We can then subquery this intermediate table and sum clicks for all days up and including the current date. This is essentially cursor style action, and can replace your use of session variables.