Why Can't I Use Aliases in the HAVING Clause in SQL?

Inability of Alias Usage in Having Clause: Logical Reasons

In SQL, an alias provides an alternative reference to a column or table to enhance readability. However, using an alias in the HAVING clause, as shown in the example code, results in an "Invalid column name" error. This occurs because the evaluation order of SQL operations dictates that the HAVING clause is processed before alias assignment.

The execution flow in SQL is as follows:

1. Table Selection: Entities mentioned in the FROM clause are combined.
2. Row Filtering: Rows not matching the WHERE clause condition are removed.
3. Grouping: Data is divided into groups based on the GROUP BY clause.
4. Group Evaluation: HAVING clause checks groups for specific criteria.
5. Column Selection: Data for output is selected as per the SELECT clause.
6. Distinctness: Duplicate rows are eliminated if specified in SELECT.
7. Sorting: Results are arranged as defined in the ORDER BY clause.

In the given scenario, the alias "col7" is assigned in the SELECT clause. However, by the time the HAVING clause is evaluated, alias assignment has not yet occurred. Thus, the reference to "col7" becomes invalid, resulting in the error.

This explanation clarifies that alias usage in the HAVING clause is restricted due to the sequence of SQL computations and emphasizes that the ORDER BY clause can utilize aliases because it is processed after alias assignment.

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