In BigQuery, the QUALIFY clause is used to filter rows after applying window functions. It works similarly to the HAVING clause but is applied after window functions rather than aggregate functions.

The QUALIFY clause allows you to filter rows based on the results of window functions. This is useful when you need to retain only those rows that meet specific conditions derived from window function results.

Let's say we have a table with sales data, and we need to find the latest sales for each salesperson. We can use the window function ROW_NUMBER() to number the rows within each group of salespeople and then use QUALIFY to select only the rows with the number 1.

Without QUALIFY, you would have to use a subquery or a Common Table Expression (CTE), which could make the query more complex and harder to read. QUALIFY simplifies such tasks, making queries more concise and easier to understand.

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