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add week2 content, homework
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‎Week2/MAKEME.md

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# Homework week 2
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## Writing a model to communicate with the TODO database
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Look at [todo_app/db.sql](./todo_app/db.sql) and use it to create a database. In
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the [todo_app/program.js](./todo_app/program.js) file there's a little program
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that should be able to extract TODOs from the database, update them, and delete
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them. The `load` function already extracts all TODOs from the database.
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Run `npm install` inside the `todo_app` directory to download and install the MySQL
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connector.
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Try to understand what happens in this program. How is the database connection
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created? How do we use the connection to query the database?
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You'll find the following empty functions in the `program.js` file, please
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implement them:
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*Read how to [use and escape query values](https://github.com/mysqljs/mysql#escaping-query-values)*
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```js
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class TodoModel {
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// ...
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create(description, callback) {
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// Write code and query to create a new TODO item
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}
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update(id, description, callback) {
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// Write code and query to update and existing TODO item
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}
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delete(id, callback) {
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// Write code and query to delete an existing TODO item
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}
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tagTodoItem(todoItemId, tagId, callback) {
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// Write code and query add a tag to a TODO item
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}
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untagTodoItem(todoItemId, tagId, callback) {
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// Write code and query remove a tag from a TODO item
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}
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markCompleted(todoItemId, callback) {
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// Write code to mark a TODO item as completed
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}
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}
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```
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# Adding a new database user
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Until now we've always connected to the database as `root`. We don't want to
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allow our TODO app access to other databases than the TODO app itself:
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- Figure out how to create a new user in MySQL.
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- Restrict the access for that user to only the `todo_app` database.
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- Use the newly created user credentials (username, password) in the connector of
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the `program.js` file.
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The homework contains 3 parts
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## Part 1 : More SQL queries through JS program
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Write a node-JS program to get answers of following queries
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on the **new_world** database: Note that you are expected to get the input from user
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and use **prepared** statements to write the queries.
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Hint: use [this] (https://github.com/mysqljs/mysql) link to read more
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about prepared statements.
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1. What is the capital of country X ? (Accept X from user)
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2. List all the languages spoken in the region Y (Accept Y from user)
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3. Find the number of cities in which language Z is spoken (Accept Z from user)
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4. Are there any countries that have
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A) Same official language
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B) Same region
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If yes, display those countries.
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If no, display TRUE or FALSE
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5. List all the continents with the number of languages spoken in each continent
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## Part 2 : SQL research
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I want to get alerts when a country has >= 10 languages.
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E.g. If a country X has 9 languages in the CountryLanguage table,
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and a user INSERTs one more row in the CountryLanguage table, then I should get an alert.
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*How can I achieve this ?*
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- Write the necessary SQL statements for this solution and
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- Test your solution with example insert statements.
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## Part 3 : Database design
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I want to develop a ToDo app in which a user can create a multiple
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ToDo lists with different purposes. Each list has at least one item.
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Each item can be tagged. Each item can be marked completed.
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There could be reminders for some items.
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- Draw an Entity Relationship Diagram for the database of the ToDo App.
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Read about Entity-relationship diagrams on the Internet
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E.g. [here] (https://www.lucidchart.com/pages/er-diagrams)

‎Week2/README.md

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# Lesson 2: Practical Database Usage
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# Lesson 2: Group by, Having and Joins. Promisification of JS client with prepared statements
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In this class, students will learn how to use more complex SQL queries to retrieve information across tables, and interact with data including write operations.
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Objective: Students should be able to build CRUD functionality using SQL statements, including INSERT INTO, UPDATE, etc.
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Objective: This class introduces more clauses (group by, having) in the
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select statement. MySQL joins (inner, self, left and right) should be explained
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with demonstration (Employee table with **reportsTo** field and Department
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table with its PK in Employee table is suitable for this demonstration).
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Promise based JavaScript program with SQL prepared statements should be
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understood students. The program can be found in the Week2 folder (Credits:
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@remarcmij
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## Pre-Class Readings
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Before arriving to class on Sunday, please watch all of the videos in [this video playlist](https://www.lynda.com/SharedPlaylist/0d62f3e4428e44ada89466cdbc296fc0) on Lynda.
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- Inserting, Updating, and Deleting
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- Understanding Write Conflicts
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- Planning Your Database
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- The Data Definition Language
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- Understanding Stored Procedures and Injection Attacks
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Also, please read the following page that explains the ACID database model.
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- [The ACID Database Model](https://www.thoughtco.com/the-acid-model-1019731)
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## Main Topics
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- INSERT INTO
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- UPDATE
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- DELETE
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- Writing SQL in your application
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- Raw SQL strings
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- Parameter validation
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- Escaping
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- Prepared statements
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- Stored procedures
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- Security
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- SQL Injection
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- User GRANTS
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- Enumeration
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## Topics to be covered
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### NOT NULL and default values in CREATE table statement
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Following links are worth reading.
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- [Working with nulls] (https://dev.mysql.com/doc/refman/8.0/en/working-with-null.html)
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- [TO DEFAULT or TO NULL] (https://blog.jooq.org/2014/11/11/have-you-ever-wondered-about-the-difference-between-not-null-and-default/)
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### foreign key
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Creating foreign key while creating the table
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```
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CREATE TABLE Employee (
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other fields,
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dept_id int,
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foreign key (dept_id)
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references Department(id)
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);
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```
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Creating the foreign key by explicitly adding the constraint
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```
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ALTER TABLE Employee ADD CONSTRAINT fk_dept foreign key (dept_id) references Department(id);
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```
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### Database dump
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A database dump (aka SQL dump) contains a record of the table structure
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and/or the data from a database and is usually in the form of a list of SQL statements.
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- Collecting the dump of an existing database from terminal `mysqldump -uroot -p database > dump-file.sql`
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- Applying the dump from mysql command prompt `source /path/to/the/dump/file`
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- Applying the dump from the terminal `mysql -uroot -p [database] < /path/to/the/dump/file`
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### Group by and Having clauses
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- *Group by* clause is used to group rows with same values.
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- It can be used in conjunction with aggregate functions (E.g. min, max).
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- The queries that contain the *group by* clause only return a single row for every grouped item.
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- *Having* clause restricts the query results of *group by* clause.
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### INSERT INTO table SET syntax
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```
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INSERT INTO Department SET dept_id=101, dept_name='fun', dept_head='unmesh';
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```
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### Promise based program demo
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### Relationships between tables : 1-M, M-M
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- One to One (one user has one profile)
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- One to Many (one department has many employees)
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- Many to Many (book(s) and author(s))
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### Adding a column to the table
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```
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alter table Employee add column dept_id int
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```
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### Update table (add a department head for a department)
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```
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update Department set dept_head = 'Lucas' where dept_id = 3;
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```
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### JOINs : CROSS, left, right, self, inner
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- A comma (,) after **FROM** is equivalent to the CROSS join.
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- Implicit inner join (when the keyword **JOIN** is not used), **WHERE** clause has conditions.
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- self join use case : Employee table with (*eid* field and *reports_to* field)
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- left and right join : reverse of each other
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- [Join manual](https://dev.mysql.com/doc/refman/8.0/en/join.html)
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## Reference Material
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‎Week2/async-create-insert.js

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const util = require('util');
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const mysql = require('mysql');
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const connection = mysql.createConnection({
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host: 'localhost',
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user: 'hyfuser',
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password: 'hyfpassword',
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database: 'class17',
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});
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// Promisify the bind function of query function of connection object
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// Pass connection object (because bind expects "this")
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// Afterwards execQuery will work as a function that returns a promise but
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// we don't have to call "then" over that promise
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const execQuery = util.promisify(connection.query.bind(connection));
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async function seedDatabase() {
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const CREATE_STUDENTS_TABLE = `
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CREATE TABLE IF NOT EXISTS students (
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student_number INT,
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student_name VARCHAR(50),
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date_of_birth DATE,
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grade FLOAT,
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gender ENUM('m', 'f')
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);`;
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const CREATE_TEACHERS_TABLE = `
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CREATE TABLE IF NOT EXISTS teachers (
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teacher_number INT,
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teacher_name VARCHAR(50),
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date_of_birth DATE,
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subject TEXT,
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gender ENUM('m', 'f')
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);`;
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const students = [
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{
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student_number: 4444,
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student_name: 'Benno',
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date_of_birth: '1995-04-26',
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grade: 8.3,
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gender: 'm',
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},
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{
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student_number: 3333,
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student_name: 'Henriata',
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date_of_birth: '1998-05-12',
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grade: 8.5,
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gender: 'm',
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},
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];
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connection.connect();
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try {
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// call the function that returns promise
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await execQuery(CREATE_STUDENTS_TABLE);
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await execQuery(CREATE_TEACHERS_TABLE);
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students.forEach(async student => {
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await execQuery('INSERT IGNORE INTO students SET ?', student);
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});
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} catch (error) {
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console.error(error);
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}
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connection.end();
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}
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seedDatabase();

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