Insert value based on previously calculated value

Question

I have the following table called 'tasks_processes':

id| task_id | process_id | start | end  
 1| 2       | 43         | 0     | 250
 2| 2       | 82         | 250   | 500
 3| 2       | 21         | 500   | 750

So, 'start' and 'end' columns represent bounds of range and are unique within the task. For new process I need perform INSERT SELECT query like this:

INSERT INTO tasks_processes (start,end,task_id,process_id) 
    SELECT 
        COALESCE ((SELECT MAX(end) FROM tasks_processes WHERE task_id=2),0) as new_val , 
        new_val + 250, 
        2, 
        12;

In other words I need to insert value based on previously calculated value. But MySQL gives me DatabaseError: no such column: new_val

How to solve this problem?

Answer 1

You have to repeat the subquery, like this:

INSERT INTO tasks_processes (start,end,task_id,process_id) 
SELECT COALESCE ((SELECT MAX(end) FROM tasks_processes WHERE task_id=2),0), 
       COALESCE ((SELECT MAX(end) FROM tasks_processes WHERE task_id=2),0) + 250, 
       2, 
       12;

or you can use something like this:

INSERT INTO tasks_processes (start,end,task_id,process_id) 
SELECT new_val, new_val+250, 2, 12
FROM (SELECT coalesce(MAX(end), 0) as new_val FROM tasks_processes WHERE task_id=2) s;