Question

I have a master table called “parent” and a related table called “childs”

Now I run a query against the master table to update some values with the sum from the child table like this.

UPDATE master m SET
    quantity1 = (SELECT SUM(quantity1) FROM childs c WHERE c.master_id = m.id),
    quantity2 = (SELECT SUM(quantity2) FROM childs c WHERE c.master_id = m.id),
    count =  (SELECT COUNT(*) FROM childs c WHERE c.master_id = m.id)
WHERE master_id = 666;

Which works as expected but is not a good style because I basically make multiple SELECT querys on the same result. Is there a way to optimize that? (Making a query first and storing the values is not an option.

I tried this:

UPDATE master m SET (quantity1, quantity2, count) = (
    SELECT SUM(quantity1), SUM(quantity2), COUNT(*)
       FROM childs c WHERE c.master_id = m.id
) WHERE master_id = 666;

but that doesn’t work.

Update: Here is the solution, thanks to everbody:

You can do something like this:

UPDATE master m
  INNER JOIN childs c ON m.master_id = c.master_id
SET master.quantity1 = c.quantity1,
    master.count = 1

If you have only one child record at a time. However if you want to use a group function like SUM() in the joined table that doesn’t work. Either you get a “Invalid use of group function” if you leave the “group by” part or a “You have an error in your sql syntax if you use “GROUP BY c.master_id”

-- This doesnt work :(
UPDATE master m
  INNER JOIN childs c ON m.master_id = c.master_id
SET master.quantity1 = SUM(c.quantity1),
    master.count = COUNT(c.*)
GROUP by c.master_id

The solution is to use JOIN with a subquery:

UPDATE master m
  INNER JOIN 
  (
    SELECT   master_id,
             SUM(quantity1) as quantity1,
             COUNT(*) as count
    FROM     childs c 
    GROUP BY master_id
  ) c
  ON c.master_id = m.master_id
SET m.quantity1 = c.quantity1,
    m.count = c.count
WHERE   m.master_id = 666;

But since this pulls every row from the childtable the overhead would likely be bigger than using more subqueries like in the original sql. So you should add a WHERE clause to the joined table to get only the rows you need.

Another interesting approach is this syntax, which does the same as the JOIN with the WHERE clause but you should only use if if you want to update all rows with the same values and your subquery only returns one row, since the result from the subquery gets appended to the result and can be used like any column.

UPDATE master m,
    (
      SELECT SUM(c.quantity1) as sum_of_quantity,
      COUNT(*) as rowcount FROM child c WHERE c.master_id = 666
    ) as c
SET m.quantity1 = c.sum_of_quantity,
    m.count = c.rowcount
WHERE m.master_id = 666;

Answer 1

Rewriting Lieven’s solution to MySQL:

UPDATE master m 
JOIN (
    SELECT  master_id
            , SUM(quantity1) as quantity1
            , SUM(quantity2) as quantity2 
            , COUNT(*) as count
    FROM    childs c 
    GROUP BY
            master_id
) c
ON c.master_id = m.master_id
SET
  m.quantity1 = c.quantity1
  ,m.quantity2 = c.quantity2
  ,m.count = c.count
WHERE   m.master_id = 666;