TASK DEFINITION: In this task you are expected to write an SQL query that will return the data asked for in the question. An SQL query works by selecting data from a table where certain conditions apply. A table contains columns where every row in that table must have a value for each column. Every table has a primary key that uniquely identifies each row, usually an id. To choose which columns are returned you specify that after the "SELECT" statement. Next, you use a "FROM" statement to specify what tables you want to select the data from. When you specify a table you can rename it with the "AS" statement. You can reference that table by whatever name follows the "AS" statement. If you want to select data from multiple tables you need to use the "JOIN" statement. This will join the tables together by pairing a row in one table with every row in the other table (Cartesian Product). To limit the number of rows returned you should use the "ON" statement. This will only return rows where the condition specified after the statement is true, this is usually an equals operator with primary keys. You can also use the "WHERE" statement to specify that only rows with column values statisfying a certain condition, should be returned. The "GROUP BY" statement will group rows together that have equal column values for whatever columns follows the statement. The "HAVING" statement will return groups that statisfy whatever condition follows the statement. Any column(s) being returned from grouped rows must either be an aggregate function, (AVG, MAX, COUNT, SUM, ...) of a column, or the column(s) that the data was grouped by. To sort the returned data you can use the "ORDER BY" command which will order the data by whatever aggregate function or column follows the statement. The "DESC" statement will sort in descending order and the "ASC" statement will sort in ascending order. Finally, you can use the "LIMIT" statement to return a certain number of rows. When "*" is used in an SQL statement every column is returned. For example, SELECT * FROM table WHERE attribute = 1, will select every column from rows with the attribute column equal to 1.
PROBLEM: What are the names of the aircraft that the least people are certified to fly?

SOLUTION: SELECT T2.name FROM Certificate AS T1 JOIN Aircraft AS T2 ON T2.aid  =  T1.aid GROUP BY T1.aid ORDER BY count(*) DESC LIMIT 1

PROBLEM: Which city is the most frequent source airport?

SOLUTION: SELECT T1.City FROM AIRPORTS AS T1 JOIN FLIGHTS AS T2 ON T1.AirportCode  =  T2.SourceAirport GROUP BY T1.City ORDER BY count(*) DESC LIMIT 1

PROBLEM: How many employees does each role have? List role description, id and number of employees.

SOLUTION:
SELECT T1.role_description ,  T2.role_code ,  count(*) FROM ROLES AS T1 JOIN Employees AS T2 ON T1.role_code = T2.role_code GROUP BY T2.role_code