Why Can't I Use Column Aliases in the Same SELECT Statement?

Explanation that column aliases in SQL queries cannot be reused in the same SELECT statement

In the given SQL query, try to use column aliases (avg_time and cnt) in an expression (ROUND(avg_time * cnt, 2)) after the SELECT statement. However, this throws an error because the column alias is not accessible in subsequent SELECT expressions.

This limitation arises from the order in which the SQL engine processes queries. The SELECT statement is executed first, and aliases are created during this phase. However, subsequent expressions are processed later, at which point the alias is not yet defined.

Solution: Use subquery

To work around this limitation, subqueries can be used. A subquery is a separate query embedded within another query. In this case, you can use subqueries to create aliases and then access them in the outer query.

The following query uses a subquery to encapsulate the original query and make column aliases available in the outer layer:

<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>

In this query, the subquery (in parentheses) creates the column aliases stddev_time, max_time, avg_time, min_time, and cnt. The outer query then selects columns from the subquery, including the alias avg_time, which is used in the expression ROUND(avg_time * cnt, 2) without encountering any errors.

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