How do I UPDATE from a SELECT in SQL Server?

To update data in SQL Server based on the results of a SELECT statement, you can use a common table expression (CTE) or a subquery. Here’s an example of how you can accomplish this:

Using a Common Table Expression (CTE):

WITH CTE AS (
  SELECT column1, column2, ...
  FROM your_table
  WHERE condition
)
UPDATE CTE
SET column1 = new_value1, column2 = new_value2, ...

Using a Subquery:

UPDATE your_table
SET column1 = new_value1, column2 = new_value2, ...
WHERE (column1, column2, ...) IN (
  SELECT column1, column2, ...
  FROM your_table
  WHERE condition
)

In both cases, you start with a SELECT statement that retrieves the rows you want to update. You can specify conditions using the WHERE clause to narrow down the selection. The SELECT statement is enclosed within parentheses and used as a derived table in the UPDATE statement.

After the SELECT statement, the UPDATE statement modifies the selected rows by setting new values for the columns using the SET clause. You can specify new values directly or use variables, expressions, or functions to calculate the new values.

Make sure to replace “your_table” with the actual name of your table, “condition” with the appropriate filtering conditions for your selection, “column1”, “column2”, etc. with the names of the columns you want to update, and “new_value1”, “new_value2”, etc. with the new values you want to set.

It’s important to note that when performing updates based on a SELECT, you should be cautious to ensure that the SELECT statement is returning the desired rows for update, as updating the wrong rows can lead to unintended consequences. Always double-check your conditions and test your queries before executing them on production data.