MySQL if cell equals this then use number, else zero

I have an SQL statement that looks like the following:

SELECT  each_req.serial_num,
        each_req.sponsors,
        each_req.subscription_code,
        format(sum(trans.trans_data-ifnull(trans.trans_prev,0))/3600,2) as hours,
        each_req.subject
        FROM   each_req,
               transactions AS trans
        WHERE  trans.serial_num = each_req.serial_num
               AND trans.type != 'priority'
               AND trans.actor LIKE 'username'
               AND trans.trans_date > unix_timestamp('2015-07-29 00:00:00')
               AND trans.trans_date < unix_timestamp('2015-07-29 14:22:03')
        GROUP BY each_req.serial_num

The problem is that the trans.trans_prev and trans.trans_data has a mix of data types based on what kind of trans.trans_type it is.

I have the line format(sum(trans.trans_data-ifnull(trans.trans_prev,0))/3600,2) as hours, in my SQL statement. I essentially want this to equal 0 if trans.trans_type != 'time_worked'.

I already have an ifnull check in there, however I am unclear how I can add an if statement that will compare against the columns data.