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.
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Question: find each value of course_id in Student_Enrolment_Courses table along with the number of the corresponding rows to each value

Answer: SELECT Count ( * ) , course_id FROM Student_Enrolment_Courses GROUP BY course_id


Question: Step 1: For each row in Person table, find the corresponding rows in PersonFriend table.
Step 2: find the summation of age of each value of friend in the results of step 1.
Step 3: find PersonFriend's name in the results of step 1 whose corresponding value in step 2 is greater than or equals engineer

Answer: SELECT T2.name FROM Person AS T1 JOIN PersonFriend AS T2 ON T1.name = T2.name GROUP BY T2.friend HAVING Sum ( T1.age ) > = "engineer"


Question: find the number of rows in Roles table

Answer:
SELECT Count ( * ) FROM Roles