How to Update Table Values Using Aggregate Functions in SQL?

Updating Table Values with Aggregate Functions in SQL

In SQL, the goal is to modify the field1 column of a table1 using an aggregate function, such as SUM, to calculate values from a related table2. However, direct implementation can encounter limitations due to unsupported operations in the SET clause.

To resolve this, an alternative approach is employed. We utilize a subquery to pre-calculate the aggregated values and then use that result as the source for the update. In this case, we calculate the sum of table2.field2 for each unique value of field3 using GROUP BY.

Here's a revised query that incorporates this technique:

<br>UPDATE t1<br>SET t1.field1 = t2.field2Sum<br>FROM table1 t1<br>INNER JOIN (</p>
<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">SELECT field3, SUM(field2) AS field2Sum
FROM table2
GROUP BY field3

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) AS t2
ON t2.field3 = t1.field3;

In this revised query:

• A subquery within parentheses calculates the sum of table2.field2 for each unique value of field3 and stores the results in a temporary table or derived table named t2.
• The outer query then joins table1 with t2 on the field3 column and updates the field1 column of table1 using the field2Sum value from t2.

This approach allows us to utilize the power of aggregate functions like SUM while updating a table, providing a flexible and efficient mechanism for manipulating data in SQL.

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