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Lecture 4 - CS50's Web Programming with Python and JavaScript.htm
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<!DOCTYPE html>
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<meta name="viewport" content="initial-scale=1, width=device-width"><meta property="og:description" content="This course picks up where Harvard University's CS50 leaves off, diving more deeply into the design and implementation of web apps with Python, JavaScript, and SQL using frameworks like Django, React, and Bootstrap. Topics include database design, scalability, security, and user experience. Through hands-on projects, students learn to write and use APIs, create interactive UIs, and leverage cloud services like GitHub and Heroku. By semester’s end, students emerge with knowledge and experience in principles, languages, and tools that empower them to design and deploy applications on the Internet.">
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</ul>
</li>
<li><a href="#tables" id="markdown-toc-tables">Tables</a></li>
<li><a href="#select" id="markdown-toc-select">SELECT</a> <ul>
<li><a href="#working-with-sql-in-the-terminal" id="markdown-toc-working-with-sql-in-the-terminal">Working with SQL in the Terminal</a></li>
<li><a href="#functions" id="markdown-toc-functions">Functions</a></li>
<li><a href="#update" id="markdown-toc-update">UPDATE</a></li>
<li><a href="#delete" id="markdown-toc-delete">DELETE</a></li>
<li><a href="#other-clauses" id="markdown-toc-other-clauses">Other Clauses</a></li>
</ul>
</li>
<li><a href="#joining-tables" id="markdown-toc-joining-tables">Joining Tables</a> <ul>
<li><a href="#join-query" id="markdown-toc-join-query">JOIN Query</a></li>
<li><a href="#indexing" id="markdown-toc-indexing">Indexing:</a></li>
<li><a href="#sql-vulnerabilities" id="markdown-toc-sql-vulnerabilities">SQL Vulnerabilities</a></li>
</ul>
</li>
<li><a href="#django-models" id="markdown-toc-django-models">Django Models</a></li>
<li><a href="#migrations" id="markdown-toc-migrations">Migrations</a></li>
<li><a href="#shell" id="markdown-toc-shell">Shell</a> <ul>
<li><a href="#starting-our-application" id="markdown-toc-starting-our-application">Starting our application</a></li>
</ul>
</li>
<li><a href="#django-admin" id="markdown-toc-django-admin">Django Admin</a></li>
<li><a href="#many-to-many-relationships" id="markdown-toc-many-to-many-relationships">Many-to-Many Relationships</a></li>
<li><a href="#users" id="markdown-toc-users">Users</a></li>
</ul>
<a data-id="" id="introduction" style="top: -81.3333px;"></a><h2><a data-id="" href="#introduction">Introduction</a></h2>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>So
far, we’ve discussed how to build simple web pages using HTML and CSS,
and how to use Git and GitHub in order to keep track of changes to our
code and collaborate with others. We also familiarized ourselves with
the Python programming language, and started using Django to create web
applications.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>Today, we’ll learn about using SQL and Django models to efficiently store and access data.</li>
</ul>
<a data-id="" id="sql" style="top: -81.3333px;"></a><h2><a data-id="" href="#sql">SQL</a></h2>
<p>: <a href="https://www.w3schools.com/sql/">SQL</a>, or Structured Query Language, is a programming language that allows us to update and query databases.</p>
<p><img src="Lecture%204%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/sql.png" alt="SQL logo"></p>
<a data-id="" id="databases" style="top: -81.3333px;"></a><h3><a data-id="" href="#databases">Databases</a></h3>
<p>Before we get into how to use the SQL language, we should discuss how our data is stored. When using SQL, we’ll work with a <a href="https://www.oracle.com/database/what-is-a-relational-database/#:~:text=A%20relational%20database%20is%20a,of%20representing%20data%20in%20tables.">relational database</a> where we can find all of our data stored in a number of <a href="https://www.essentialsql.com/what-is-a-database-table/">tables</a>. Each of these tables is made up of a set number of columns and a flexible number of rows.</p>
<p>To illustrate how to work with SQL, we’ll use the example of a
website for an airline used to keep track of flights and passengers. In
the following table, we see that we’re keeping track of a number of
flights, each of which has an <code class="language-plaintext highlighter-rouge">origin</code>, a <code class="language-plaintext highlighter-rouge">destination</code>, and a <code class="language-plaintext highlighter-rouge">duration</code>.</p>
<p><img src="Lecture%204%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/flights0.png" alt="Flights 0"></p>
<p>There are several different relational database management systems
that are commonly used to store information, and that can easily
interact with SQL commands:</p>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.mysql.com/">MySQL</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.postgresql.org/">PostgreSQL</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.sqlite.org/index.html">SQLite</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>…</li>
</ul>
<p>The first two, MySQL and PostgreSQL, are heavier-duty database
management systems that are typically run on servers separate from those
running a website. SQLite, on the other hand, is a lighter-weight
system that can store all of its data in a single file. We’ll be using
SQLite throughout this course, as it is the default system used by
Django.</p>
<a data-id="" id="column-types" style="top: -81.3333px;"></a><h3><a data-id="" href="#column-types">Column Types</a></h3>
<p>Just as we worked with several different variable types in Python, SQLite has <a href="https://www.sqlite.org/datatype3.html">types</a>
that represent different forms of information. Other management systems
may have different data types, but all are fairly similar to those of
SQLite:</p>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><code class="language-plaintext highlighter-rouge">TEXT</code>: For strings of text (Ex. a person’s name)</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><code class="language-plaintext highlighter-rouge">NUMERIC</code>: A more general form of numeric data (Ex. A date or boolean value)</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><code class="language-plaintext highlighter-rouge">INTEGER</code>: Any non-decimal number (Ex. a person’s age)</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><code class="language-plaintext highlighter-rouge">REAL</code>: Any real number (Ex. a person’s weight)</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><code class="language-plaintext highlighter-rouge">BLOB</code> (Binary Large Object): Any other binary data that we may want to store in our database (Ex. an image)</li>
</ul>
<a data-id="" id="tables" style="top: -81.3333px;"></a><h2><a data-id="" href="#tables">Tables</a></h2>
<p>Now, to actually get started with using SQL to interact with a database, let’s begin by creating a new table. The <a href="https://www.w3schools.com/sql/sql_create_table.asp">command to create a new table</a> looks something like this:</p>
<div class="language-sql highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">CREATE</span> <span class="k">TABLE</span> <span class="n">flights</span><span class="p">(</span>
<span class="n">id</span> <span class="nb">INTEGER</span> <span class="k">PRIMARY</span> <span class="k">KEY</span> <span class="n">AUTOINCREMENT</span><span class="p">,</span>
<span class="n">origin</span> <span class="nb">TEXT</span> <span class="k">NOT</span> <span class="k">NULL</span><span class="p">,</span>
<span class="n">destination</span> <span class="nb">TEXT</span> <span class="k">NOT</span> <span class="k">NULL</span><span class="p">,</span>
<span class="n">duration</span> <span class="nb">INTEGER</span> <span class="k">NOT</span> <span class="k">NULL</span>
<span class="p">);</span>
</code></pre></div></div>
<p>In the above command, we’re creating a new table that we’ve decided to call <code class="language-plaintext highlighter-rouge">flights</code>, and we’ve added four columns to this table:</p>
<ol>
<li><code class="language-plaintext highlighter-rouge">id</code>: It is often helpful to have an number that allows us to uniquely identify each row in a table. Here we have specified that <code class="language-plaintext highlighter-rouge">id</code> is an integer, and also that it is our <a href="https://www.w3schools.com/sql/sql_primarykey.ASP">primary key</a>, meaning it is our unique identifier. We have additionally specified that it will <code class="language-plaintext highlighter-rouge">AUTOINCREMENT</code>, which means we will not have to provide an id every time we add to the table because it will be done automatically.</li>
<li><code class="language-plaintext highlighter-rouge">origin</code>: Here we’ve specified that this will be a text field, and by writing <code class="language-plaintext highlighter-rouge">NOT NULL</code> we have required that it have a value.</li>
<li><code class="language-plaintext highlighter-rouge">destination</code>: Again we’ve specified that this will be a text field and prevented it from being null.</li>
<li><code class="language-plaintext highlighter-rouge">duration</code>: Again this value cannot be null, but this time it is represented by an integer rather than as text.</li>
</ol>
<p>We just saw the <code class="language-plaintext highlighter-rouge">NOT NULL</code> and <code class="language-plaintext highlighter-rouge">PRIMARY KEY</code> constraint when making a column, but there are several other <a href="https://www.tutorialspoint.com/sqlite/sqlite_constraints.htm">constraints</a> available to us:</p>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><code class="language-plaintext highlighter-rouge">CHECK</code>: Makes sure certain constraints are met before allowing a row to be added/modified</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><code class="language-plaintext highlighter-rouge">DEFAULT</code>: Provides a default value if no value is given</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><code class="language-plaintext highlighter-rouge">NOT NULL</code>: Makes sure a value is provided</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><code class="language-plaintext highlighter-rouge">PRIMARY KEY</code>: Indicates this is the primary way of searching for a row in the database</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><code class="language-plaintext highlighter-rouge">UNIQUE</code>: Ensures that no two rows have the same value in that column.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>…</li>
</ul>
<p>Now that we’ve seen how to create a table, let’s look at how we can add rows to it. In SQL, we do this using the <code class="language-plaintext highlighter-rouge">INSERT</code> command:</p>
<div class="language-sql highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">INSERT</span> <span class="k">INTO</span> <span class="n">flights</span>
<span class="p">(</span><span class="n">origin</span><span class="p">,</span> <span class="n">destination</span><span class="p">,</span> <span class="n">duration</span><span class="p">)</span>
<span class="k">VALUES</span> <span class="p">(</span><span class="nv">"New York"</span><span class="p">,</span> <span class="nv">"London"</span><span class="p">,</span> <span class="mi">415</span><span class="p">);</span>
</code></pre></div></div>
<p>In the above command, we’ve specified the table name we wish to
insert into, then provided a list of the column names we will be
providing information on, and then specified the <code class="language-plaintext highlighter-rouge">VALUES</code> we would like to fill that row in the table, making sure the <code class="language-plaintext highlighter-rouge">VALUES</code> come in the same order as our corresponding list of columns. Note that we don’t need to provide a value for <code class="language-plaintext highlighter-rouge">id</code> because it is automatically incrementing.</p>
<a data-id="" id="select" style="top: -81.3333px;"></a><h2><a data-id="" href="#select">SELECT</a></h2>
<p>Once a table has been populated with some rows, we’ll probably want a
way to access data within that table. We do this using SQL’s <a href="https://www.w3schools.com/sql/sql_select.asp">SELECT</a> query. The simplest <code class="language-plaintext highlighter-rouge">SELECT</code> query into our flights table might look something like this:</p>
<div class="language-sql highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">SELECT</span> <span class="o">*</span> <span class="k">FROM</span> <span class="n">flights</span><span class="p">;</span>
</code></pre></div></div>
<p>The above command (*) retrieves all of the data from our flights table</p>
<p><img src="Lecture%204%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/all.png" alt="all"></p>
<p>It may be the case though that we don’t really need all of the
columns from the database, just origin and destination. To access just
these columns, we can replace the * with the column names we would like
access to. The following query returns all of the origins and
destinations.</p>
<div class="language-sql highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">SELECT</span> <span class="n">origin</span><span class="p">,</span> <span class="n">destination</span> <span class="k">FROM</span> <span class="n">flights</span><span class="p">;</span>
</code></pre></div></div>
<p><img src="Lecture%204%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/flights1.png" alt="Just two cols"></p>
<p>As our tables get larger and larger, we will also want to narrow down which rows our query returns. We do this by adding a <a href="https://www.w3schools.com/sql/sql_where.asp">WHERE</a> followed by some condition. For example, the following command selects only row with an <code class="language-plaintext highlighter-rouge">id</code> of <code class="language-plaintext highlighter-rouge">3</code>:</p>
<div class="language-sql highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">SELECT</span> <span class="o">*</span> <span class="k">FROM</span> <span class="n">flights</span> <span class="k">WHERE</span> <span class="n">id</span> <span class="o">=</span> <span class="mi">3</span><span class="p">;</span>
</code></pre></div></div>
<p><img src="Lecture%204%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/where0.png" alt="only one row"></p>
<p>e can filter by any column, not just <code class="language-plaintext highlighter-rouge">id</code>!</p>
<div class="language-sql highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">SELECT</span> <span class="o">*</span> <span class="k">FROM</span> <span class="n">flights</span> <span class="k">WHERE</span> <span class="n">origin</span> <span class="o">=</span> <span class="nv">"New York"</span><span class="p">;</span>
</code></pre></div></div>
<p><img src="Lecture%204%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/where1.png" alt="Origin is New York"></p>
<a data-id="" id="working-with-sql-in-the-terminal" style="top: -81.3333px;"></a><h3><a data-id="" href="#working-with-sql-in-the-terminal">Working with SQL in the Terminal</a></h3>
<p>Now that we know some basic SQL commands, let’s test them out in the
terminal! In order to work with SQLite on your computer, you must first <a href="https://www.sqlite.org/download.html">download SQLite</a>. (We won’t use it in lecture, but you can also <a href="https://sqlitebrowser.org/dl/">download DB Browser</a> for a more user-friendly way to run SQL queries.)</p>
<p>We can start by creating a file for our database either by manually creating a new file, or running <code class="language-plaintext highlighter-rouge">touch flights.sql</code> in the terminal. Now, if we run <code class="language-plaintext highlighter-rouge">sqlite3 flights.sql</code> in the terminal, we’ll be brought to a SQLite prompt where we can run SQL commands:</p>
<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
<span class="c"># Entering into the SQLite Prompt</span>
<span class="o">(</span>base<span class="o">)</span> % sqlite3 flights.sql
SQLite version 3.26.0 2018-12-01 12:34:55
Enter <span class="s2">".help"</span> <span class="k">for </span>usage hints.
<span class="c"># Creating a new Table</span>
sqlite> CREATE TABLE flights<span class="o">(</span>
...> <span class="nb">id </span>INTEGER PRIMARY KEY AUTOINCREMENT,
...> origin TEXT NOT NULL,
...> destination TEXT NOT NULL,
...> duration INTEGER NOT NULL
...> <span class="o">)</span><span class="p">;</span>
<span class="c"># Listing all current tables (Just flights for now)</span>
sqlite> .tables
flights
<span class="c"># Querying for everything within flights (Which is now empty)</span>
sqlite> SELECT <span class="k">*</span> FROM flights<span class="p">;</span>
<span class="c"># Adding one flight</span>
sqlite> INSERT INTO flights
...> <span class="o">(</span>origin, destination, duration<span class="o">)</span>
...> VALUES <span class="o">(</span><span class="s2">"New York"</span>, <span class="s2">"London"</span>, 415<span class="o">)</span><span class="p">;</span>
<span class="c"># Checking for new information, which we can now see</span>
sqlite> SELECT <span class="k">*</span> FROM flights<span class="p">;</span>
1|New York|London|415
<span class="c"># Adding some more flights</span>
sqlite> INSERT INTO flights <span class="o">(</span>origin, destination, duration<span class="o">)</span> VALUES <span class="o">(</span><span class="s2">"Shanghai"</span>, <span class="s2">"Paris"</span>, 760<span class="o">)</span><span class="p">;</span>
sqlite> INSERT INTO flights <span class="o">(</span>origin, destination, duration<span class="o">)</span> VALUES <span class="o">(</span><span class="s2">"Istanbul"</span>, <span class="s2">"Tokyo"</span>, 700<span class="o">)</span><span class="p">;</span>
sqlite> INSERT INTO flights <span class="o">(</span>origin, destination, duration<span class="o">)</span> VALUES <span class="o">(</span><span class="s2">"New York"</span>, <span class="s2">"Paris"</span>, 435<span class="o">)</span><span class="p">;</span>
sqlite> INSERT INTO flights <span class="o">(</span>origin, destination, duration<span class="o">)</span> VALUES <span class="o">(</span><span class="s2">"Moscow"</span>, <span class="s2">"Paris"</span>, 245<span class="o">)</span><span class="p">;</span>
sqlite> INSERT INTO flights <span class="o">(</span>origin, destination, duration<span class="o">)</span> VALUES <span class="o">(</span><span class="s2">"Lima"</span>, <span class="s2">"New York"</span>, 455<span class="o">)</span><span class="p">;</span>
<span class="c"># Querying this new information</span>
sqlite> SELECT <span class="k">*</span> FROM flights<span class="p">;</span>
1|New York|London|415
2|Shanghai|Paris|760
3|Istanbul|Tokyo|700
4|New York|Paris|435
5|Moscow|Paris|245
6|Lima|New York|455
<span class="c"># Changing the settings to make output more readable</span>