1. Computer Science 101 Web Access to Databases SQL – Extended Form

2. SQL - Aggregate Functions Aggregate Functions –Count –Sum –Max –Min –Avg Get birth date of oldest student SELECT MIN(Birthdate) FROM Students

3. SQL Query - Expanded Form SELECT FROM [WHERE ] [GROUP BY ] [HAVING ] [ORDER BY ]

4. SQL - Expanded Form (cont.) SELECT: attributes or functions retrieved in result (columns of result) FROM: all tables needed to do query WHERE: conditions for selecting rows

5. SQL - Expanded Form (cont.) GROUP BY: attributes for grouping - one row in result for each group value HAVING: conditions for selecting group rows ORDER BY: order for display, can specify ASC or DESC

6. SQL - Group By Note: It must be that all attributes listed in the Select clause are constant within each group specified by the Group By clause. For example, we could not say SELECT LastName, ClassYear FROM Students GROUP BY ClassYear since the LastName varies within a ClassYear Usually, the GROUP BY clause is used with aggregate functions to get statistics on the groups.

7. SQL - Group By examples Actually, the SELECT and GROUP BY lists should be the same except for aggregate functions in the Select list Give a table of counts by class

8. SQL - Group By examples Restrict to current students and sort by count (descending) and ClassYear. Give a table of counts by class

9. SQL - Group By examples (cont.) Give a list of faculty names together with the number of their advisees.

10. SQL - Group By examples (cont.) Just current term

11. SQL - Group By examples (cont.) List of NFL teams and the number of students expressing interest in that team

12. SQL - Having Clause The Having clause plays a role for group rows much like the Where clause plays for select rows. i.e., the Having clause specifies criteria which must be met by a group row in order for it to be in the result table.

13. SQL - Having Clause (cont.) The from clause gives a set of potential rows for the result – every combination of rows from the tables. The where clause gives a filter for these potential rows – which do we keep. The select clause specifies the columns we want. The group by clause “squashes” groups of rows into single group rows. The having clause gives a filter for these group rows.

14. SQL - Having clause examples List of interests, their categories and number of students with the interest where there are at least ten students with the interest, arranged in order of popularity, most popular first.

15. SQL - Having clause examples Just this term (3 or more):