non-equi join更好理解的方法

window function大家可能都能明白，也很好用。我看到大神有种用non-equi join的方法也可以得解，而且很普遍。

SELECT s1.emp_no, s1.salary,
    (SELECT SUM(s2.salary)
    FROM salaries s2
    WHERE s2.emp_no <= s1.emp_no
    AND s2.to_date = '9999-01-01') 
FROM salaries s1
WHERE s1.to_date = '9999-01-01'
ORDER BY s1.emp_no；

其实我是不太理解这种方法的，是如何直接在select里面就取出来值，欢迎大佬来解释一下，我也用non-equi的方法写出了一个解，更加好理解一点，发出来和大家交流一下。

SELECT s1.emp_no, s1.salary, SUM(s2.salary)  AS running_total
FROM salaries s1, salaries s2
WHERE s1.emp_no >= s2.emp_no
AND s1.to_date = '9999-01-01'
AND s2.to_date = '9999-01-01'
GROUP BY s1.emp_no
ORDER BY s1.emp_no；