Aggregate function in an SQL update query?

Solution 1:

UPDATE t1
SET t1.field1 = t2.field2Sum
FROM table1 t1
INNER JOIN (select field3, sum(field2) as field2Sum
   from table2
  group by field3) as t2
on t2.field3 = t1.field3  

Solution 2:

Use:

UPDATE table1
   SET field1 = (SELECT SUM(t2.field2) 
                   FROM TABLE2 t2 
                  WHERE t2.field3 = field2)

Solution 3:

Or you could use a mix of JBrooks and OMG Ponies answers:

UPDATE table1
   SET field1 = (SELECT SUM(field2)
                   FROM table2 AS t2
                  WHERE t2.field3 = t1.field3)
  FROM table1 AS t1

Solution 4:

A good situation to use CROSS APPLY

UPDATE t1
   SET t1.field1 = t2.field2Sum
  FROM table1 t1
 CROSS APPLY (SELECT SUM(field2) as field2Sum
                FROM table2 t2
               WHERE t2.field3 = t1.field3) AS t2

Solution 5:

I know the question is tagged SQL Server but be careful with UPDATE with JOIN if you are using PostgreSQL. @JBrooks answer won't work :

UPDATE t1
SET t1.field1 = t2.field2Sum
FROM table1 t1
INNER JOIN (...) as t2
on t2.field3 = t1.field3  

You will have to adapt it to :

UPDATE table1 t1
SET t1.field1 = t2.field2Sum
FROM (...) as t2
WHERE t2.field3 = t1.field3  

See parameter from_list in the doc to get why FROM is considered by PostgreSQL as a self-join : https://www.postgresql.org/docs/9.5/static/sql-update.html#AEN89239