Can I Use Column Aliases in Subsequent Expressions Within the Same SQL SELECT Statement?

The use of column aliases in SQL SELECT statements in subsequent expressions

In SQL, you may encounter situations where you want to use column aliases in subsequent expressions in the same SELECT statement. However, trying to do this may result in an error similar to the one described in the question.

The reason for this limitation is that column aliases are processed together when the SELECT statement is evaluated. Therefore, aliases are not available within the same SELECT statement.

Solution: Encapsulate the query in a subquery

To solve this problem, you can encapsulate the original query in a subquery. This creates a new scope in which the column alias can be used outside the subquery.

Here's how to modify the query to use the column aliases avg_time and cnt in the expression ROUND(avg_time * cnt, 2):

<code class="language-sql">SELECT stddev_time, max_time, avg_time, min_time, cnt, 
       ROUND(avg_time * cnt, 2) as slowdown
FROM (
        SELECT 
            COALESCE(ROUND(stddev_samp(time), 2), 0) as stddev_time, 
            MAX(time) as max_time, 
            ROUND(AVG(time), 2) as avg_time, 
            MIN(time) as min_time, 
            COUNT(path) as cnt, 
            path
        FROM 
            loadtime
        GROUP BY
            path
        ORDER BY
            avg_time DESC
        LIMIT 10
   ) X;</code>

By encapsulating the query in a subquery, the alias can be used in an outer SELECT statement, allowing you to successfully evaluate the expression ROUND(avg_time * cnt, 2).

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