Write a SQL query to find all pairs of rows (a, b) such that a is in table A, b is in table B, and a and b have the same value for column C.
This technical problem involves writing a SQL query to find the average salary of employees by department. The example input table includes columns for department, first name, last name, and salary. The expected output is a table with columns for department and average salary.
Given two tables, "A" and "B", write a SQL query to find all the rows from table "A" that have no corresponding rows in table "B".