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assign2.md

+## Project 2: Advanced SQL, Python DB app
+
+### Setup
+
+Download the startup files [here](https://ceres.cs.umd.edu/424/assign/assignment2Dist.tgz).
+
+Build (`docker build --rm -t 424 .`) and run (`docker run -it -v $(pwd):/424 424`)
+the container as before, then `cd /424` (inside the container).
+The main differences here are that we have loaded a skewed database
+(`flightsskewed`), and also installed Java. 
+
+Ensure that the `flightsskewed` database has been created, together with tables populated from  `large-skewed.sql`. Use
+`flightsskewed` for all parts of this assignment.
+
+## Part 1: Query Construction (2 pts)
+
+Consider the following query which finds the number of flights
+  taken by users whose name starts with 'William'.
+
+```
+select c.customerid, c.name, count(*)
+from customers c join flewon f on (c.customerid = f.customerid and c.name like 'William%')
+group by c.customerid, c.name
+order by c.customerid;
+```
+
+The result however does not contain the users whose name contains 'William' but who did
+not fly at all (e.g., `cust733`). So we may consider
+modifying this query to use a left outer join instead, so we get those users as well:
+
+```
+select c.customerid, c.name, count(*)
+from customers c left outer join flewon f on (c.customerid = f.customerid and c.name like 'William%')
+group by c.customerid, c.name
+order by c.customerid;
+```
+
+Briefly explain why this query does not return the expected answer (as below), and rewrite the query so that it does.
+
+The final answer should look like this:
+```
+	customerid |              name              | count
+	------------+--------------------------------+-------
+	cust727    | William Harris                 |     4
+	cust728    | William Hill                   |     6
+	cust729    | William Jackson                |     6
+	cust730    | William Johnson                |     5
+	cust731    | William Lee                    |     0
+	cust732    | William Lopez                  |     6
+	cust733    | William Martinez               |     0
+	cust734    | William Mitchell               |     6
+	cust735    | William Moore                  |     5
+	cust736    | William Parker                 |     4
+	cust737    | William Roberts                |     8
+	cust738    | William Robinson               |     7
+	cust739    | William Rodriguez              |     5
+	cust740    | William Wright                 |     8
+	cust741    | William Young                  |     5
+	(15 rows)
+```
+
+Save your query in  `queries.py` as the definition of `queryWilliam`.
+Include your explanation as a comment above this definition.
+
+---
+## Part 2: Trigger (3 pt)
+
+We have built a table `NumberOfFlightsTaken(customerid, customername,
+numflights)` to keep track of the total number of flights taken by each
+customer:
+```
+create table NumberOfFlightsTaken as
+select c.customerid, c.name as customername, count(*) as numflights
+from customers c join flewon fo on c.customerid = fo.customerid
+group by c.customerid, c.name;
+```
+
+Since this is a derived table (and not a view), it will not be kept
+up-to-date by the database system.  We (you) will therefore
+write a `trigger` to keep this new table updated when a new entry is inserted
+into, or a row is deleted from, the `flewon` table. Remember that the `customerid`
+corresponding to a new `flewon` insertion update may not yet exist in the
+`NumberOfFlightsTaken` table. In that case, it should be added to `NumberOfFlightsTaken`
+with a count of 1. 
+
+Similarly, if deletion of a row in `flewon`
+results in a user not having any flights, then the corresponding tuple for
+that user in `NumberOfFlightsTaken` should be deleted. 
+
+The trigger code should be submitted as the definition of `queryTrigger` in
+the `queries.py` file, as straight SQL. This file has an incorrect and
+incomplete version of such a trigger commented out. Uncomment this version,
+fix it, and test by running `SQLTesting.py`. 
+
+Look inside this file to see
+the insertions and deletions being tested, and think about what the proper
+actions should be.
+
+Notes:
+- `python3 SQLTesting.py` will clean the db, set up `NumberOfFlightsTaken`,
+  and run both your queries, printing their outputs.
+- We will again be using automated testing. Ensuring that your queries looks to
+  be generating correct data w/ `SQLTesting.py` should ensure that the
+  autograder will produce correct results as well.
+
+### Non-VM instructions (e.g. Macs)
+
+- Create db w/ `createdb flightsskewed`.
+- Ensure that the *user* in SQLTesting.py matches your user account.
+
+## Submission
+
+Submit the `queries.py` file [on gradescope](https://www.gradescope.com/courses/811728/assignments/4669976/review_grades).
+