Pass 1z0-071 Exam Guide

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.

In Oracle 12c SQL, the GROUP BY clause is used to arrange identical data into groups with the GROUP BY expression followed by the SELECT statement. The SUM() function is then used to calculate the sum for each grouped record on a specific column, which in this case is the salary column.

Option A is valid because it correctly applies the GROUP BY clause. Both dept_id and join_date are included in the SELECT statement, which is a requirement when using these columns in conjunction with the GROUP BY clause. This means that the query will calculate the sum of salaries for each combination of dept_id and join_date. It adheres to the SQL rule that every item in the SELECT list must be either an aggregate function or appear in the GROUP BY clause.

Option B is invalid due to a typo in SELECT depe_id and also because it ends with a colon rather than a semicolon.

Option C is invalid because you cannot nest aggregate functions like MAX(AVG(salary)) without a subquery.

Option D is invalid for the same reason as option C, where it tries to nest aggregate functions AVG(MAX(salary)), which is not allowed directly in SQL without a subquery.

For further reference, you can consult the Oracle 12c documentation, which provides comprehensive guidelines on how to use the GROUP BY clause and aggregate functions like SUM():

• Oracle Database SQL Language Reference, 12c Release 1 (12.1): GROUP BY Clause
• Oracle Database SQL Language Reference, 12c Release 1 (12.1): Aggregate Functions

A: True. TRUNCATE is generally faster than DELETE for removing all rows from a table because TRUNCATE is a DDL (Data Definition Language) operation that minimally logs the action and does not generate rollback information. TRUNCATE drops and re-creates the table, which is much quicker than deleting rows one by one as DELETE does, especially for large tables. Also, TRUNCATE does not fire triggers.

References:

• Oracle documentation specifies that TRUNCATE is faster because it doesn't generate redo logs for each row as DELETE would.
• TRUNCATE cannot be rolled back once executed, since it is a DDL command and does not generate rollback information as DML commands do.

The scenario requires prompting for column names and WHERE condition each time the query is executed, but only prompting for the table name once. This behavior is achievable through the use of substitution variables in SQL*Plus or SQL Developer:

• A.

SELECT &col1, &col2 FROM &&table WHERE &condition;

This query uses & for columns and condition, which will prompt every time the query is run. &&table will prompt for the table name the first time and reuse the same value in the same session without re-prompting.

Options B, C, D, and E use incorrect syntax or variable types, leading to various errors or undesired behaviors, such as not maintaining the value of the table name across executions or incorrect usage of single quotes and comparison in SQL.