Which function should you use to get the last updated row for each customer in a T-SQL query?

To retrieve the last updated row for each customer in a T-SQL query, the function that is most appropriate is ROW_NUMBER(). This function assigns a unique sequential integer to rows within a partition of a result set, allowing you to filter down to the most recent entry for each customer.

Using ROW_NUMBER(), you can order the data by the update timestamp in descending order and partition the data by customer. This way, the most recent updates are assigned the rank of 1. With a subsequent filtering step, you can select only those rows where the rank is 1, effectively isolating the latest update for each customer.

The other functions, while useful in different contexts, do not serve this specific purpose. LAST_VALUE() tends to return the last value in an ordered set but does not allow for filtering unique rows per group directly. MAX() can find the maximum value of a designated column but does not return entire rows and is not designed for sequential ordering. SUM() aggregates numeric values and similarly does not deal with row retrieval, making it unsuitable for obtaining specific row data based on conditions like the latest updates.