SQL Update to the SUM of its joined values

I'm trying to update a field in the database to the sum of its joined values:

UPDATE P
SET extrasPrice = SUM(E.price)
FROM dbo.BookingPitchExtras AS E
INNER JOIN dbo.BookingPitches AS P ON E.pitchID = P.ID
    AND P.bookingID = 1
WHERE E.[required] = 1

When I run this I get the following error:

"An aggregate may not appear in the set list of an UPDATE statement."

Any ideas?

How about this:

UPDATE p
SET p.extrasPrice = t.sumPrice
FROM BookingPitches AS p
INNER JOIN
    (
        SELECT PitchID, SUM(Price) sumPrice
        FROM BookingPitchExtras
        WHERE [required] = 1
        GROUP BY PitchID 
    ) t
    ON t.PitchID = p.ID
WHERE p.bookingID = 1

An alternate to the above solutions is using Aliases for Tables:

UPDATE T1 SET T1.extrasPrice = (SELECT SUM(T2.Price) FROM BookingPitchExtras T2 WHERE T2.pitchID = T1.ID)
FROM BookingPitches T1;

I ran into the same issue and found that I could solve it with a Common Table Expression (available in SQL 2005 or later):

;with cte as (
	SELECT PitchID, SUM(Price) somePrice
    FROM BookingPitchExtras
    WHERE [required] = 1 
    GROUP BY PitchID)
UPDATE p SET p.extrasPrice=cte.SomePrice
FROM BookingPitches p INNER JOIN cte ON p.ID=cte.PitchID
WHERE p.BookingID=1

This is a valid error. See this. Following (and others suggested below) are the ways to achieve this:-

UPDATE P 
SET extrasPrice = t.TotalPrice
FROM BookingPitches AS P INNER JOIN
 (
  SELECT
    PitchID,
    SUM(Price) TotalPrice
  FROM
     BookingPitchExtras
  GROUP BY PitchID
  ) t
ON t.PitchID = p.ID

You need something like this :

UPDATE P
SET ExtrasPrice = E.TotalPrice
FROM dbo.BookingPitches AS P
INNER JOIN (SELECT BPE.PitchID, Sum(BPE.Price) AS TotalPrice
	FROM BookingPitchExtras AS BPE
	WHERE BPE.[Required] = 1
	GROUP BY BPE.PitchID) AS E ON P.ID = E.PitchID
WHERE P.BookingID = 1

With postgres, I had to adjust the solution with this to work for me:

UPDATE BookingPitches AS p
SET extrasPrice = t.sumPrice
FROM 
    (
        SELECT PitchID, SUM(Price) sumPrice
        FROM BookingPitchExtras
        WHERE [required] = 1
        GROUP BY PitchID 
    ) t
WHERE t.PitchID = p.ID AND p.bookingID = 1

Use a sub query similar to the below.

UPDATE P
SET extrasPrice = sub.TotalPrice from
BookingPitches p
inner join 
(Select PitchID, Sum(Price) TotalPrice
	from  dbo.BookingPitchExtras
	Where [Required] = 1
	Group by Pitchid
) as Sub
on p.Id = e.PitchId 
where p.BookingId = 1