MySQL 8.0

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CREATE TABLE status
    (`Date` varchar(10), `isDone` int)
;
    
INSERT INTO status
    (`Date`, `isDone`)
VALUES
    ('2018-10-01', 1),
    ('2018-10-02', 1),
    ('2018-10-03', 1),
    ('2018-10-04', 1),
    ('2018-10-10', 0),
    ('2018-10-15', 1),
    ('2018-10-16', 0),
    ('2018-10-18', 1),
    ('2018-10-19', 1),
    ('2018-10-20', 1)
;

Records: 10  Duplicates: 0  Warnings: 0


SELECT s.Date,
       @streak := IF(Date = @last_date + INTERVAL 1 DAY AND isDone = 1, @streak+1, 1) AS streak,
       @last_date := Date AS last_date
FROM status s
JOIN (SELECT @streak := 0, @last_date := '1900-01-01') i
ORDER BY s.Date



Date
streak
last_date


2018-10-01
1
2018-10-01

2018-10-02
2
2018-10-02

2018-10-03
3
2018-10-03

2018-10-04
4
2018-10-04

2018-10-10
1
2018-10-10

2018-10-15
1
2018-10-15

2018-10-16
1
2018-10-16

2018-10-18
1
2018-10-18

2018-10-19
2
2018-10-19

2018-10-20
3
2018-10-20


SELECT Date, streak
FROM (SELECT s.Date,
       @streak := IF(Date = @last_date + INTERVAL 1 DAY AND isDone = 1, @streak+1, 1) AS streak,
       @last_date := Date AS last_date
FROM status s
JOIN (SELECT @streak := 0, @last_date := '1900-01-01') i
ORDER BY s.Date) s
WHERE s.Date = '2018-10-20'


Date
streak


2018-10-20
3

Date	streak	last_date
2018-10-01	1	2018-10-01
2018-10-02	2	2018-10-02
2018-10-03	3	2018-10-03
2018-10-04	4	2018-10-04
2018-10-10	1	2018-10-10
2018-10-15	1	2018-10-15
2018-10-16	1	2018-10-16
2018-10-18	1	2018-10-18
2018-10-19	2	2018-10-19
2018-10-20	3	2018-10-20