多个左连接导致错误总和

I got a MySQL Database where I store ship employees and their job experience. I got one table where I store the employees, one table where I store their previous jobs in the shipping department ( Type of Job , Years ) and another one table where I store their other previous jobs ( Type of Hob , Years ). All these are connected using the ID of the employee. When I fetch an employee I'd like to get the sum of his shipping and general experience, but the select query returns wrong sum for the general experience. I know this is cause by the multiple joins and because the tables are not joined properly, but I am not sure on how to fix this. Could you please help me? I got a MySQL Database where I store ship emplo