SQL 5.7 Lead Function - mysql-workbench

Question

I'm struggling emulating a lead function to calculate the difference of (after date - current date)
I'm currently using mysql 5.7 to accomplish this. I have tried looking at various sources on stack overflow but I'm not sure how to get the result.
This is what I want:
What I currently have now is the same thing without the days column.
I would also like to know how to get a column of dates that grabs the date after the current date.

Answer 1

This seems to work (except for the unclear row=4):
DROP TABLE IF EXISTS table4;
CREATE TABLE table4 (id integer, user_id integer, product varchar(10), `date` date);
INSERT INTO table4 VALUES
(1,1,'item1','2020-01-01'),
(2,1,'item2','2020-01-01'),
(3,1,'item3','2020-01-02'),
(4,1,'item4','2020-01-02'),
(5,2,'item5','2020-01-06'),
(6,2,'item6','2020-01-09'),
(7,2,'item7','2020-01-09'),
(8,2,'item8','2020-01-10');
SELECT
id,
user_id,
product,
date,
(SELECT date FROM table4 t4 WHERE t4.id>t1.id LIMIT 1) x,
COALESCE(DATEDIFF((SELECT date FROM table4 t4 WHERE t4.id>t1.id LIMIT 1),date),0) as days
FROM table4 t1
output:
+ ------- + ------------ + ------------ + --------- + ----------- + --------- +
| id | user_id | product | date | x | days |
+ ------- + ------------ + ------------ + --------- + ----------- + --------- +
| 1 | 1 | item1 | 2020-01-01 | 2020-01-01 | 0 |
| 2 | 1 | item2 | 2020-01-01 | 2020-01-02 | 1 |
| 3 | 1 | item3 | 2020-01-02 | 2020-01-02 | 0 |
| 4 | 1 | item4 | 2020-01-02 | 2020-01-06 | 4 |
| 5 | 2 | item5 | 2020-01-06 | 2020-01-09 | 3 |
| 6 | 2 | item6 | 2020-01-09 | 2020-01-09 | 0 |
| 7 | 2 | item7 | 2020-01-09 | 2020-01-10 | 1 |
| 8 | 2 | item8 | 2020-01-10 | | 0 |
+ ------- + ------------ + ------------ + ---------- + ---------- + --------- +
The column x is only here for to see which date is returned from the subquery, and not really needed for the final result.
DBFIDDLE
EDIT: when there are no "gaps" in the numbering of id, you could do this to get a solution which should have more performance:
SELECT
t1.id,
t1.user_id,
t1.product,
t1.date,
COALESCE(DATEDIFF(t2.date,t1.date),0) as days
FROM table4 t1
LEFT JOIN table4 t2 on t2.id = t1.id+1
I added this to the DBFIDDLE