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SET @a = '2016-12-14 13:00:00'; /* NEW START DATE */
SET @b = '2016-12-14 15:00:00'; /* NEW END DATE */

SELECT * FROM bot_db.tasks
WHERE chat_id = 81182535
AND schedule_id = 1
AND start_date < @b
AND end_date > @a

ALTER TABLE  `your_table_name` ADD  `id` INT NOT NULL AUTO_INCREMENT PRIMARY KEY FIRST;