TASK DEFINITION: In this task you are expected to provide an SQL statement from an english description of what that SQL statement does. The description may include multiple steps but you should only ouput one SQL statement that accomplishes every step. 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: Step 1: For each row in station table, find the corresponding rows in status table.
Step 2: find the average bikes_available of each value of station_id in the results of step 1.
Step 3: find name, id in the results of step 1 whose corresponding value in step 2 is greater than 14

SOLUTION: SELECT T1.name , T1.id FROM station AS T1 JOIN status AS T2 ON T1.id = T2.station_id GROUP BY T2.station_id HAVING Avg ( T2.bikes_available ) > 14

PROBLEM: Step 1: find the Name of Scientists table.
Step 2: find the Name of Scientists table whose Name not one of the results of step 1

SOLUTION: SELECT Name FROM Scientists WHERE Name NOT IN ( SELECT Name FROM Scientists )

PROBLEM: Step 1: find the StuID of Has_Pet table.
Step 2: find without repetition the Fname, Age of Student table whose Student's StuID not one of the results of step 1

SOLUTION:
SELECT DISTINCT T1.Fname , T1.Age FROM Student AS T1 WHERE T1.StuID NOT IN ( SELECT T2.StuID FROM Has_Pet AS T2 )