<p>I'd like to make an (MS)SQL query that returns something like this:</p>

<pre><code>Col1 Col2                  Col3
---- --------------------- ------
AAA  18.92                 18.92
BBB  20.00                 40.00
AAA  30.84                 30.84
BBB  06.00                 12.00
AAA  30.84                 30.84
AAA  46.79                 46.79
AAA  86.40                 86.40
</code></pre>

<p>where Col3 is equal to Col2 when Col1 = AAA and Col3 is twice Col2 when Col1 = BBB.
Can someone point me in the rigth direction please?</p>

You didn't mention what kind of database you're using.  Here's something that will work in SQL Server:
```
```
SELECT Col1, Col2, 
    CASE WHEN Col1='AAA' THEN Col2 WHEN Col1='BBB' THEN Col2*2 ELSE NULL END AS Col3
FROM ...
```
```