SELECT

    invoice.invoice_id,
    IF(invoice.paid=0,'unbilled',IF(invoice.paid=1,'paid','unpaid')) AS status,
    IFNULL(SUM(tasks.task_hours),0) AS project_hours,
    IFNULL( FORMAT(SUM(tasks.fee),2),0) AS project_fee
    
FROM

    invoice LEFT JOIN invoice_task ON invoice.invoice_id = invoice_task.invoice_id
    LEFT JOIN tasks ON invoice_task.task_id = tasks.task_id
    
WHERE

    invoice.project_id='##project_id##'
   
GROUP BY

    invoice.invoice_id