use mysql SUM() in a WHERE clause

suppose I have this table

id | cash 
1    200
2    301
3    101
4    700

and I want to return the first row in which the sum of all the previous cash is greater than a certain value:

So for instance, if I want to return the first row in which the sum of all the previous cash is greater than 500, is should return to row 3

How do I do this using mysql statement?

using WHERE SUM(cash) > 500 doesn't work

You can only use aggregates for comparison in the HAVING clause:

GROUP BY ...
  HAVING SUM(cash) > 500

The HAVING clause requires you to define a GROUP BY clause.

To get the first row where the sum of all the previous cash is greater than a certain value, use:

SELECT y.id, y.cash
  FROM (SELECT t.id,
               t.cash,
               (SELECT SUM(x.cash)
                  FROM TABLE x
                 WHERE x.id <= t.id) AS running_total
         FROM TABLE t
     ORDER BY t.id) y
 WHERE y.running_total > 500
ORDER BY y.id
   LIMIT 1

Because the aggregate function occurs in a subquery, the column alias for it can be referenced in the WHERE clause.

Not tested, but I think this will be close?

SELECT m1.id
FROM mytable m1
INNER JOIN mytable m2 ON m1.id < m2.id
GROUP BY m1.id
HAVING SUM(m1.cash) > 500
ORDER BY m1.id
LIMIT 1,2

The idea is to SUM up all the previous rows, get only the ones where the sum of the previous rows is > 500, then skip one and return the next one.

In general, a condition in the WHERE clause of an SQL query can reference only a single row. The context of a WHERE clause is evaluated before any order has been defined by an ORDER BY clause, and there is no implicit order to an RDBMS table.

You can use a derived table to join each row to the group of rows with a lesser id value, and produce the sum of each sum group. Then test where the sum meets your criterion.

CREATE TABLE MyTable ( id INT PRIMARY KEY, cash INT );

INSERT INTO MyTable (id, cash) VALUES
  (1, 200), (2, 301), (3, 101), (4, 700);

SELECT s.*
FROM (
  SELECT t.id, SUM(prev.cash) AS cash_sum
  FROM MyTable t JOIN MyTable prev ON (t.id > prev.id)
  GROUP BY t.id) AS s
WHERE s.cash_sum >= 500
ORDER BY s.id
LIMIT 1;

Output:

+----+----------+
| id | cash_sum |
+----+----------+
|  3 |      501 |
+----+----------+