Как вывести 0 если нет данных в mysql?

SELECT t1.calldate, 
    t2.answered, 
    t3.no_answer 
FROM 
    (SELECT DISTINCT(DATE(calldate)) AS calldate 
     FROM calls c
    ) AS t1
    INNER JOIN (SELECT DISTINCT(DATE(calldate)) AS calldate, 
                    COUNT(ID) AS answered 
                FROM calls 
                WHERE disposition = 1 
                    AND did = '555' 
                    AND MONTH(calldate) = 08  
                    AND YEAR(calldate) = 2017 
                GROUP BY 1
               ) t2 ON t1.calldate = t2.calldate
    INNER JOIN (SELECT DISTINCT(DATE(calldate)) AS calldate, 
                    COUNT(ID) AS no_answer 
                FROM calls 
                WHERE disposition <> 1 
                    AND did = '555' 
                    AND MONTH(calldate) = 08 
                    AND YEAR(calldate) = 2017  
                GROUP BY 1
               ) AS t3 ON t1.calldate = t3.calldate;

Используйте IFNULL:

IFNULL(t3.no_answer, "null")

или IFNULL(t3.no_answer, 0)

http://www.mysqltutorial.org/mysql-ifnull/

SELECT DATE(calldate) AS calldate,
        SUM(    disposition = 1 
            AND did = '555' 
            AND MONTH(calldate) = 08  
            AND YEAR(calldate) = 2017) AS answered,
        SUM(    disposition <> 1 
            AND did = '555' 
            AND MONTH(calldate) = 08 
            AND YEAR(calldate) = 2017) AS no_answer
FROM calls
GROUP BY DATE(calldate)

Сам разобрался) Проблема была в том, что я использовал полное соединение (INNER JOIN) вместо LEFT JOIN, в том месте где нет данных)

 SELECT t1.calldate, t2.answered, IFNULL(t3.no_answer,0) AS no_answer FROM(SELECT DISTINCT(DATE(calldate)) AS calldate FROM calls c) AS t1
 INNER JOIN(SELECT DISTINCT(DATE(calldate)) AS calldate, COUNT(ID) AS answered FROM calls WHERE disposition = 1 AND did = '555' AND MONTH(calldate) = 08  AND YEAR(calldate) = 2017 GROUP BY 1) t2 ON t1.calldate = t2.calldate
 LEFT JOIN(SELECT DISTINCT(DATE(calldate)) AS calldate, COUNT(ID) AS no_answer FROM calls WHERE disposition <> 1 AND did = '555' AND MONTH(calldate) = 08 AND YEAR(calldate) = 2017  GROUP BY 1) AS t3 ON t1.calldate = t3.calldate;

Как минимум, третий запрос не нужен, можно сделать через sum подсчёт количества. Как я понял, для каждой даты вам нужно количество звонков. INNER JOIN соединяет только там, где есть звонки с указанными условиями. Используйте LEFT JOIN с ifnull в итоговом select:

SELECT t1.calldate, 
    ifnull(t2.answered, 0) as answered, 
    ifnull(t2.no_answer, 0) as no_answer
FROM 
    (SELECT DISTINCT(DATE(calldate)) AS calldate 
     FROM calls c
    ) AS t1
    LEFT JOIN (SELECT DISTINCT(DATE(calldate)) AS calldate, 
                    SUM(if(disposition = 1, 1, 0)) AS answered,
                    SUM(if(disposition <> 1, 1, 0)) as no_answer
                FROM calls 
                WHERE did = '555' 
                    AND MONTH(calldate) = 08  
                    AND YEAR(calldate) = 2017 
                GROUP BY 1
               ) t2 ON t1.calldate = t2.calldate