How Can I Use Aggregate Functions Like SUM in an SQL UPDATE Query?

Aggregate Function in an SQL Update Query

The question seeks to understand how aggregate functions, such as SUM, can be utilized in SQL update queries. The provided example attempts to update a value in a table based on the sum of values from another table. However, it encounters the limitation that SET statements do not support SUM and GROUP BY.

To resolve this issue, the solution employs a subquery to calculate the sum and then use the result in the update query. The following code snippet demonstrates the corrected approach:

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  

In this corrected query:

1. A subquery is defined to calculate the sum of field2 for each unique value of field3 in table2. The result is stored in a temporary table or view named t2.
2. The update query uses a JOIN to merge table1 with t2 based on the common column field3.
3. The SET statement updates the field1 column in table1 to the corresponding field2Sum value calculated in the subquery.

By employing a subquery, this approach effectively addresses the limitations of SET statements and allows for the use of aggregate functions in update queries.

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