我的php脚本中有一个mysql查询，目前运行良好。唯一的问题是有些列返回空值。
如果有这些列的数据，则返回值，但如果没有日期的数据或记录，则返回null。我所要做的就是修改这个查询，以便如果有任何内容为null，它将返回“0”。
我不确定是否应该使用ifnull或coalesce，但无论哪种方法，我都不熟悉将其应用于此查询的最佳方法。
非常感谢您的帮助。

$stmt3 = mysqli_prepare($conn2,
     "UPDATE ambition.ambition_totals a
        INNER JOIN 
        (SELECT 
            c.user AS UserID,
            COUNT(*) AS dealers,
            ROUND((al.NumberOfDealers / al.NumberOfDealerContacts) * 100 ,2)  AS percent
        FROM jfi_dealers.contact_events c
        JOIN jackson_id.users u
        ON c.user = u.id
        JOIN jfi_dealers.dealers d
        ON c.dealer_num = d.dealer_num
        LEFT JOIN (
          SELECT user_id, COUNT(*) AS NumberOfDealerContacts,
          SUM(CASE WHEN ( d.next_call_date + INTERVAL 7 DAY) THEN 1 ELSE 0 END) AS NumberOfDealers
          FROM jackson_id.attr_list AS al
          JOIN jfi_dealers.dealers AS d ON d.csr = al.data
          WHERE al.attr_id = 14
          GROUP BY user_id) AS al
        ON al.user_id = c.user
        GROUP BY UserID) as cu
        on cu.UserID = a.ext_id 
        SET a.dealers_contacted = cu.dealers,
          a.percent_up_to_date = cu.percent;
                ") or die(mysqli_error($conn2));

更新
带有ifnull语句的版本：

UPDATE ambition.ambition_totals a
        INNER JOIN 
        (SELECT 
            c.user AS UserID,
            ifnull(count(*),0) AS dealers,
            ifnull(ROUND((al.NumberOfDealers / al.NumberOfDealerContacts) * 100 ,2),0)  AS percent
        FROM jfi_dealers.contact_events c
        JOIN jackson_id.users u
        ON c.user = u.id
        JOIN jfi_dealers.dealers d
        ON c.dealer_num = d.dealer_num
        LEFT JOIN (
          SELECT user_id, COUNT(*) AS NumberOfDealerContacts,
          SUM(CASE WHEN ( d.next_call_date + INTERVAL 7 DAY) THEN 1 ELSE 0 END) AS NumberOfDealers
          FROM jackson_id.attr_list AS al
          JOIN jfi_dealers.dealers AS d ON d.csr = al.data
          WHERE al.attr_id = 14
          GROUP BY user_id) AS al
        ON al.user_id = c.user
        WHERE c.created_at >= CURDATE()
        GROUP BY UserID) as cu
        on cu.UserID = a.ext_id 
        SET a.dealers_contacted = cu.dealers,
          a.percent_up_to_date = cu.percent;