Perform line offset(lead Function) by making the last line 0 instead of 'NULL'

Question

I’m using correlated subsettes to simulate the LEAD function because my Mysql does not support this function, by scrolling the rows of a column. However, when moving, either through the original LEAD function or the code below that I implemented, the last line has NULL value, making it impossible for me to perform the subtraction operation.

The code is (taken from:https://dba.stackexchange.com/questions/187933/simulate-effective-lead1-or-lead2-in-mysql):

SELECT t1.digit, t2.digit,t1.digit -t2.digit as 'diff'
FROM (
    SELECT digit, @rownum1 := @rownum1 + 1 AS id
    FROM
     alfabet  , (SELECT @rownum1 := 0) r
    ORDER BY
     alfabet.digit ASC
) AS t1
LEFT OUTER JOIN ( 
    SELECT digit, @rownum2 := @rownum2 + 1 AS id
    FROM
     alfabet  , (SELECT @rownum2 := 0) r
    ORDER BY
     alfabet.digit ASC
) AS t2
  ON t1.id +1 = t2.id;

This works perfectly for me, but I have as a result:

digit   next_digit     diff
4       2              2         
2       2              0           
2       2              0          
2       1              1                  
1       NULL           NULL 

I need to make the value of 0 appear in the next_digit column instead of NULL, so that it is possible to subtract and return it to diff (1-0 = 1). Or, some way to subtract with NULL and still return the value 1 in the diff column, some operation that allows to do (1-null = 1).

[RESOLVED]

Using the coalesce function

SELECT t1.name, t1.cnt, t2.cnt_lead, coalesce (t1.cnt - t2.cnt_lead, t1.cnt) as 'diff'
``