Examine the description of the EMPLOYEES table:Examine this query:Which line produces an error?

In the provided SQL query, the issue arises from the alias 'a.avg_sal' which is defined in the subquery but is being referenced in the SELECT list of the outer query. This is not permitted in SQL as the scope of the alias defined in the subquery is only within that subquery.

Here is the breakdown of the code and the error:

Line 1: Correct syntax for initiating a SELECT statement.

Line 2: Refers to 'e.salary', which is a correct reference to the 'salary' column using alias 'e' for the employees table.

Line 3: 'a.avg_sal' attempts to reference an alias that is defined in the subquery within the outer query, which is not allowed. This is because 'avg_sal' is defined in the subquery's SELECT list and cannot be referenced outside of it. The correct way to include the average salary from the subquery in the SELECT list of the main query would be to repeat the subquery or to use a join that includes the average salary.

Line 5-7: The subquery itself is correctly formed; it computes the average salary for the same department.

Line 8: The ORDER BY clause is properly referencing 'e.last_name', which is defined in the outer query.

Therefore, the error occurs at Line 3 where 'a.avg_sal' is not a valid reference in the SELECT list of the main query because it is defined in the subquery.

The rules of scope for aliases in subqueries are specified in the Oracle Database SQL Language Reference 12c documentation. Subquery aliases cannot be referenced outside their subquery.