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1CREATE TABLE IF NOT EXISTS overlap (
2    id INT AUTO_INCREMENT,
3    start_date DATE,
4    end_date DATE,
5    PRIMARY KEY (id)
6)  ENGINE=INNODB; 
7     
8SELECT SUM(PERIOD_DIFF(EXTRACT(YEAR_MONTH FROM a.end_date), EXTRACT(YEAR_MONTH FROM a.start_date))) months
9  FROM (
10    SELECT MIN(g.start_date) start_date, MAX(g.end_date) end_date 
11      FROM (
12        SELECT @group_id := @group_id + (@end_date IS NULL OR o.start_date > @end_date) group_id,
13               start_date,
14               @end_date := DATE(CASE 
15                 WHEN (@end_date IS NULL OR o.start_date > @end_date) THEN o.end_date
16                 ELSE GREATEST(o.end_date, @end_date)
17               END) end_date  
18          FROM overlap o
19          JOIN (SELECT @group_id := 0, @end_date := NULL) init
20      ORDER BY o.start_date ASC  
21            ) g
22  GROUP BY  g.group_id  
23        ) a