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<h1>Introduction to SQL</h1>
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<p class="intro">SQL is a standard computer language for accessing and
manipulating databases.</p>
<hr />
<h2>What is SQL?</h2>
<ul>
<li>SQL stands for <b>S</b>tructured <b>Q</b>uery <b>L</b>anguage</li>
<li>SQL allows you to access a database</li>
<li>SQL is an ANSI standard computer language</li>
<li> SQL can execute queries against a database</li>
<li>SQL can retrieve data from a database</li>
<li>SQL can insert new records in a database</li>
<li>SQL can delete records from a database</li>
<li>SQL can update records in a database</li>
<li>SQL is easy to learn</li>
</ul>
<hr />
<h2>SQL is a Standard - BUT....</h2>
<p>SQL is an ANSI (American National Standards Institute) standard computer
language for accessing and manipulating database
systems. SQL statements are used to retrieve and update data in a database. SQL works with database programs like MS Access, DB2, Informix, MS SQL Server,
Oracle, Sybase, etc.</p>
<p>Unfortunately, there are many different versions of the SQL language, but to be in
compliance with the ANSI standard, they must support the same
major keywords in a similar manner (such as SELECT, UPDATE, DELETE, INSERT,
WHERE, and others).</p>
<p><b>Note:</b> Most of the SQL database programs also have their own
proprietary extensions in addition to the SQL standard!</p>
<hr />
<h2>SQL Database Tables</h2>
<p>A database most often contains one or more tables. Each table is identified
by a name (e.g. "Customers" or "Orders"). Tables contain records (rows) with
data.</p>
<p>Below is an example of a table called "Persons":</p>
<table class="ex" cellspacing="0" border="1" width="100%">
<tr>
<th align="left">LastName</th>
<th align="left">FirstName</th>
<th align="left">Address</th>
<th align="left">City</th>
</tr>
<tr>
<td>Hansen</td>
<td>Ola</td>
<td>Timoteivn 10</td>
<td>Sandnes</td>
</tr>
<tr>
<td>Svendson</td>
<td>Tove</td>
<td>Borgvn 23</td>
<td>Sandnes</td>
</tr>
<tr>
<td>Pettersen</td>
<td>Kari</td>
<td>Storgt 20</td>
<td>Stavanger</td>
</tr>
</table>
<p>
The table above contains three records (one for each person) and four columns (LastName, FirstName, Address, and City).</p>
<hr />
<h2> SQL Queries
</h2>
<p>With SQL, we can query a database and have a result set returned.
</p>
<p>A query like this:
</p>
<table class="ex" cellspacing="0" border="1" width="50%">
<tr>
<td><pre>SELECT LastName FROM Persons</pre></td>
</tr>
</table>
<p>Gives a result set like this:
</p>
<table class="ex" cellspacing="0" border="1" width="50%">
<tr>
<th align="left">LastName</th>
</tr>
<tr>
<td>Hansen</td>
</tr>
<tr>
<td>Svendson</td>
</tr>
<tr>
<td>Pettersen</td>
</tr>
</table>
<p>
<b>Note</b>: Some database systems require a semicolon at the end of the SQL statement.
We don't use the semicolon in our tutorials.</p>
<hr />
<h2>SQL Data Manipulation Language (DML)</h2>
<p>SQL (Structured Query Language) is a syntax for executing queries. But the SQL
language also includes a syntax to update, insert, and delete
records.</p>
<p>These query and update commands together form the Data Manipulation Language (DML)
part of SQL:</p>
<ul>
<li><b>SELECT</b> - extracts data from a database table</li>
<li><b>UPDATE</b> - updates data in a database table</li>
<li><b>DELETE</b> - deletes data from a database table</li>
<li><b>INSERT INTO</b> - inserts new data into a database table</li>
</ul>
<hr />
<h2>SQL Data Definition Language (DDL)</h2>
<p>The Data Definition Language (DDL) part of SQL permits database tables to be
created or deleted. We can also define indexes (keys), specify links between tables,
and impose constraints between database tables.</p>
<p>
The most important DDL statements in SQL are: </p>
<ul>
<li><b>CREATE TABLE</b> - creates a new database table</li>
<li><b>ALTER TABLE </b>- alters (changes) a database table</li>
<li><b>DROP TABLE</b> - deletes a database table</li>
<li><b>CREATE INDEX</b> - creates an index (search key)</li>
<li><b>DROP INDEX</b> - deletes an index
</li>
</ul>
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