Is there any software, preferably free, preferably supporting drag & drop, preferably supporting save & load to ease the creation diagrams like these? 

<img src="https://www.sqlite.org/images/syntax/select-stmt.gif" width="300" />

[<img src="https://i.sstatic.net/ps4uU.png" width="300" />][1]  
<sub>(source: [tikilive.org](http://v4manual.tikilive.org/wp-content/uploads/2011/07/connecting-video-source-1.png))</sub>  

 [1]: https://i.sstatic.net/ps4uU.png