Find average amount of time from Requisition Submitted to Order Created - mysql

Question

I have two tables requisition_headers and order_headers. I am interested in finding the average time it takes from the time the requisition is submitted (requisition_headers.submitted_at) and the time the order is created (orders.headers_created_at) where the requisition_headers.status <> 'draft'.
I would like the result to look like:
Avg_Req_To_PO_Cycle_Time = 3.2 Days
I have the following script but it's not working:
SELECT Database() as Customer,
AVG(timestampdiff(requisition_headers.submitted_at,order_headers.created_at)) AS REQ_PO_Cycle_Time
FROM order_headers
LEFT JOIN requisition_headers ON order_headers.requisition_header_id = requisition_headers.id
WHERE requisition_headers.status <> 'draft'
Any Ideas?
--UPDATE--
I changed the query to the following and now get a response of 229491.71 my question is- is that days, hours, minutes, seconds?
SELECT DATABASE() AS CUSTOMER,
AVG(TIME_TO_SEC(TIMEDIFF(order_headers.created_at,requisition_headers.submitted_at))) as Cycle_time
FROM order_headers LEFT JOIN requisition_headers ON order_headers.requisition_header_id = requisition_headers.id
where requisition_headers.status <> 'draft'

Answer 1

Make sure you know what a system function returns when you use it. The TimestampDiff function returns the difference between the two dates in the unit you specify in the first argument. You don't specify that unit so I don't know what you get back. I get a compile error.
In your second attempt, you are using TimeDiff which returns an interval value, then converting the result of Avg to seconds. So if you want the result in fractional days just divide by the number of seconds in a day.
You also use a left join when getting the dates. At first I thought you wanted to get all the requisitions whether the orders had been created or not. But you are joining the tables in the wrong order for that. But, assuming that is your intention, if the order has not yet been created you will be putting NULL as one of the parameters. You will get a NULL as an answer so you get nothing. If you want to use a left join, then you should specify a substitute date for any missing Created dates -- after getting the table in the right order, that is.
Here are two options. One ignores orders that have not yet been created by using a regular inner join. The other includes those but substitutes the current date and time.
By asking for the number of minutes between the dates, the final answer in days is found by dividing by the number of minutes in a day.
SQLFiddle
SELECT Customer,
AVG( timestampdiff( minute, r.submitted_at,
o.created_at)) / (24 * 60 )AS REQ_PO_Cycle_Time
FROM requisition_headers r
JOIN order_headers o
ON o.requisition_header_id = r.id
WHERE r.status <> 'draft'
group by Customer;
SELECT Customer,
AVG( timestampdiff( minute, r.submitted_at,
IfNull( o.created_at, CurDate()))) / (24 * 60 )AS REQ_PO_Cycle_Time
FROM requisition_headers r
LEFT JOIN order_headers o
ON o.requisition_header_id = r.id
WHERE r.status <> 'draft'
group by Customer;